This is a 40 amp hour battery that I plan to use in place of the 92 amp hour AGM marine deep cycle battery that I use when camping. While my old AGM battery did what it was supposed to, it does have its limitations:
First, since it's a lead acid battery, one can only use about half of the battery's capacity. Dischanging it lower than that would significantly reduce its cycle life.
Second, once the battery is discharged there's no quick way to charge it back up. The maximum recommended charge rate is C/3. This limits charging to 30 amps. At that rate it would take over 1.5 hours to charge the battery when it's down to 50%.
Third, like other lead acid batteries, AGM batteries are heavy. Mine weights 63 pounds.
LTO (lithium titanate) solves many of the problems inherent to AGM / lead acid batteries:
- The maximum charge rate is C10. For a 40ah battery, this is comes out to 400 amps!
- One can use most of the available capacity without significantly reducing the battery's cycle life.
- Storing it at a partial state of charge doesn't damage the battery in any way.
- While LTO batteries have a lower specific energy than other lithium battery chemistries, it's still significantly better than lead acid.
But there are some drawbacks:
- They're expensive.
- They're not available, off the shelf, at a local store.
- Some of the parts need to be purchased from overseas.
Sourcing the battery cells
This was the easy part. LTO batteries are popular with car audio enthusiasts. DIY Central Shop is a US-based supplier that sells a-grade Yinlong cells. They weren't cheap, but I knew I'd get what was advertised. I ordered 6 cells and 5 busbars. They arrived in a couple of days.
Balancing the cells
While Lithium Titanate batteries are among the safest of lithium-ion batteries, they still can be damaged by over-charging, and over-discharging. This can easily happen if one or more of the battery cells is charged significantly above or below the rest. The easiest and safest way to take care of this is by using a battery balancer.
Unfortunately, LTO battery balancers aren't very common. The balancer that I wanted was only available from one US-based supplier, and they were out of stock. With no other viable options, I went ahead and ordered the balancer from China then manually balanced the battery cells while I was waiting for it to arrive.
Manually balancing the battery is mainly a matter of wiring the cells in parallel, using resistors to limit the current. The picture above shows the first 4 cells (of 6 total). I started by using a 3 ohm resistor. Then I used a coil that had a series resistance of about 0.8 ohms. Since the voltages were fairly close, and since not much current was going through the resistors, I eventually ended up connecting the batteries directly together in parallel. By that point, the current being transferred was only about 0.5 amps.
While I plan on keeping the battery cells dry, they will be used in a high-humidity environment. This makes the topic of galvanic corrosion worth considering. When dis-similar metals are in contact with each other, in the presence of an electrolyte (like water + impurities), it creates a miniature battery causing one of the metals to corrode. The solution is use metals that have a similar galvanic potential. This chart shows the galvanic potential of various combinations of metals. A general rule of thumb is to keep the galvanic potential below 0.45v when operating in a high-humidity environment.
The battery cells that have aluminum connections, the bus-bars are aluminum, and the nuts are zinc-plated steel. Zinc + aluminum has a galvanic potential of 0.33v, which is within reasonable limits. However, once you start adding wires there's a problem to contend with. Wire is typically either copper or tin-plated copper. Ring terminals are usually tin-plated copper. Looking at the various combinations of metals, the following possibilities are:
- Zinc + aluminum: 0.33v
- Tin + aluminum: 0.28v
- Tin + zinc: 0.60v
- Copper + aluminum: 0.53v
- Copper + zinc: 0.85v
As you can see, zinc isn't compatible with much of anything, other than aluminum. Copper and aluminum aren't compatible with each other, but both are compatible with tin. My solution was to replace the zinc nuts with aluminum nuts, use tin-plated copper ring-terminals, and use tin-plated copper wire. This leaves two metals to work with, instead of four, and both are compatible with each other.
Attaching the busbars
The two pictures above show how I attached the busbars. This can be done in any arrangement you like, as long as it creates a series circuit. The main battery connections are shown in the lower-left and upper-right corners of the top picture. Torque spec is critical. The Yinlong cells have a torque spec of 7 newton meters, or 62 inch pounds. The is a lot lower than one would expect. I definitely recommend using a torque wrench, if you have one available to use. I used aluminum lock washers in order to prevent the nuts from loosening up. If you decide to use lock washers, I strongly recommend using flat washers underneath them. If you don't, the nuts will be very difficult to remove if you ever need to disassemble the battery pack.
As you can imagine, the battery cells are very easy to short out. Also, the busbars were the only thing holding the battery cells in place. To solve these problems, I decided the make a frame using PVC trim board. The battery pack is about 7 7/8" wide by 5 1/4" deep. The studs on the LTO cells stick up about 0.9". With that in mind, I bought a piece of trim board that is 5.5" wide by 1" thick. It comes in 10' length boards. While this may seem like an aweful lot of extra material, it actually was a good thing, considering that things don't always turn out on the first try.
I cut the top and bottom pieces of trim board to a length of 13". This allows extra room for battery terminals, while still allowing it to fit inside my group 27 battery box. Through trial and error I found that it works best to make the bottom piece first. Due to the way that the busbars are arranged it makes it easier for everything to come out level.
Making a template for the bottom piece:
I used cardboard from an old cereal box. I pushed the batteries and the cardboard into a corner, to prevent them from moving around. The battery studs have a concave shape which makes it easy to punch holes, through the cardboard, in the right places.
Marking the holes:
I took a 13" piece of PVC trim board, centered the cardboard, then marked the holes.
Below is a picture of the bottom piece.
Since each battery terminal has a bus bar connection, each one needs a large cutout to account for that. The diameter at the end of the busbars is about 1.4", I used a 1.5" forstner bit.
To clear the washers and nuts on the battery terminals, one needs a diameter of at least 0.85". I started with a normal 1" forstner bit, then finished the bottom using an old 1" forstner bit that I ground the tip off of.
Making a router template for the busbars:
Through trial and error, I found that it's worth making a router template. Without a template, it's extremely difficult to get consistent results.
I used a 1 1/4" holesaw to cut two circles out of a scrap piece of trim board, then ground them down so that they fit the battery stud holes.
I made the rest of the template out of 1" pieces of trim board, then used PVC pipe cement to glue it all together.
I used 1/2" a pattern router bit for this.
Here, I'm using a router to finish the busbar cut-outs.
This is how the bottom piece turned out, after making the busbar cutouts.
Making a template for the top piece:
I made a template, for the top, using a piece of junk mail.
Here, I marked the holes for the top piece.
While the top piece didn't turn out as "nice" as the bottom piece, the problem is mainly cosmetic. The busbars don't touch the trim board, anyway. For this reason, it was ok to use the top as-is.
I added some foam camper weatherstrip to help cushion the cells.
First I traced the cells.
Then I used a roll of electrical tape as a pattern.
While the foam doesn't look pretty, it does the job.
Next, I cut out the side pieces and used construction screws to attach the top and bottom.
The LTO cells use battery studs that are an odd size, and because they're made out of aluminum, they're easy to damage. Rather than connect to them directly, I decided to mount 3/8" feed-trough connectors to the top of the trim board frame.
Since the feed-through connectors are just under an inch in length, I started by using a 1 1/2" forstner bit to remove about 1/4" worth of material.
The feed-through connectors require a 7/8" hole. While I used a forstner bit, a hole saw would work, too.
I attached the feed-through connectors (using #10 x 3/4" sheet metal screws), attached a battery cable, then traced around it. While I hadn't planned on running the cable as a loop, it was necessary in order to accommodate the length of the battery lugs. If one were mass-producing these, custom busbars would be the way to go. Since this is a on-off project for myself, loops of battery cable will suffice.
I used a router to make the cutouts for the battery cables, freehand, using a straight bit, then used a file to smooth out the rough edges.
Finishing the busbar cutouts:
Since the M6 aluminum screws that I ordered were too long, I cut them with a side cutter then used a die to clean up the threads.
I crimped a ring terminal to an inline fuse holder then attached it to the busbar. Then I marked the wire cutout for the top piece.
After using a straight router bit to cut out a notch for the wire, I marked the busbar screw, then drilled for the screwhead, using a 7/8" forstner bit.
This picture shows the top piece with all of the cutouts complete.
The battery balancer turned out to be a lot larger than I thought it would be. To accomodate its size, I cut out a piece of PVC trim board that I would later attach to the front.
I used a 1/2" a pattern router bit to mill out an area to accomodate the battery balancer. After two failed attempts, on the 3rd try, I finally had something that was usable.
I used #8 x 1 3/4" construction screws to mount the board which houses the battery balancer.
I used #6 x 1/2" pan head sheet metal screws, along with #6 nylon washers, to attach the battery balancer.
Next, I crimped ring terminals to inline fuse holders and attached them to the bottom bus bars.
Finally, I used a straight router bit to make notches to accommodate the fuseholder wires shown in the picture above.
After attaching the bottom, I added short lengths of battery cable then added fuse holders to the negative and positive sides.
After attaching the top, I attached the battery cables to the feed-through connectors.
The battery balancer is wired, going left to right, from most negative to most positive. My original intention was to have the main negative terminal on the left and the main positive on the right. Not sure how I ended up with those flipped around. Because of that, the wiring was more of a bird's nest than it would have otherwise been. (Even though it doesn't look like it, on the picture, the black negative wire is on the far left side of the green connector.) In addition to hooking up the battery balancer, I also wired up two 6s balance cables and two white wires for later. The balance cables can be used with a cell monitor and/or a balance charger. The white wires will go to a low-voltage alarm / buzzer.
Here's what my cell monitor shows when I first plugged it in. The cells are at the lower end of their voltage range. That's because I hadn't charged them yet. Since the cell monitor doesn't have a lithium titanate setting on it, the battery percentage reading is useless. The battery definitely isn't close to being fully discharged.
These two pictures show the power cables hooked up. I used 10 gauge marine duplex cable to power small / medium sized loads, along with Anderson Powerpole connectors. I also used 2 gauge battery cable and a SMH SY175 breakaway connector for larger loads. While the Powerpole connectors use tin plated contacts, the SY175's have silver plated contacts. Alhough silver is an excellent conductor of electricty, it's not galvanically compatible with tin. To mitigate this issue I soldered the wires to the contacts, went over top of them with adhesive-lined heatshrink, then put a protective boot over that. All three power cables are fused. I used 30 amp ATO fuses for the two smaller wires, and a 150 amp "Mega" fuse for the larger wire. While there are many different kinds of fuses to choose from, I narrowed it down to what I can purchase locally. If / when I blow a fuse, I don't want to be waiting a week for a replacement to arrive.
As a test, I used jumper cables to connect the battery my car then started the engine. As it turns out, even through the alternator is rated for 140 amps, it only puts out around 30 amps at idle. When not under load, the alternator has a voltage of around 14.5 volts. The fact that the voltage is running lower than that indicates that the battery isn't the limiting factor. If left charging long enough, the 14.5v alternator voltage would limit how much the battery can be charged, though.
Since LTO isn't a common battery chemistry, suitable chargers are hard to come by. I decided to use a Mean Well RS-750-15 power supply. The main features I was looking for were adjustable voltage, and current limiting. Power supplies, like these, don't include any cables. I used a 16 gauge appliance cord, 4 gauge tinned copper battery cable, and a SY175 connector. Although the datasheet doesn't make it clear, #10 ring terminals are needed in order to attach the battery cables.
While the power supply works, as a battery charger, there are some issues to deal with. First, if you attach the battery prior to plugging in the power supply, there is no power output. Second the default current limit is 60 amps, which is above what the power supply is rated for.
Here is what the power supply's stock CN50 plug looks like. The purple wire connects pins 7 and 8. The orange wire connects pins 13 and 14. The current can be limited to the 100% rating by supplying 5 volts to pin 7. Then by running a switch between pins 13 and 14, one can control whether the power supply's output is on or off.
Instead of cutting up the CN50 plug, I ordered a new housing and some premade jumper wires. The part number for the housing is DF11-14DS-2C. The part number for the jumper wires is H3BBT-10104-B4.
Like with the stock plug, I connected pins 1 & 2, 3 & 4, 5 & 6, and 13 & 14. Pin 7 is connected to the +5v output of a 5 volt DC/DC converter. The ground wire is connected to pin 11.
Charging the battery:
Getting the charging process started is little more complicated than I would like it to be. The first step is to plug the power cord into the wall. The second step it to plug in the battery connector. The third and final step is to connect pin 13 and 14. At the time the picture was taken I was using a yellow jumper wire for this. (I ordered a switch, but it hadn't arrived yet.) Due to a quirk in the power supply, it delivers no power if the first and second steps are done in reverse order.
Here I'm confirming that applying 5 volts to pin 7 did have an effect on the output current.
While the stock power supply voltage is 15v, it is adjustable. In theory, a 6S LTO battery can be charged to 16.8v. In practice, the law of diminishing returns kicks in long before that. It gets to the point where small amounts of power cause the cell voltage to rapidly rise. This can happen faster than the battery balancer can compensate. After charging to 15v, I bumped the voltage to 16v. At 16 volts, cell #1's voltage quickly rose above 2.8v, while the others were at around 2.6v. Through experimentation I found that 15.45v was the sweet spot. At that voltage, none of the cell voltages rose above 2.8v.
To test the battery capacity, I wired a power meter inline with my power inverter. Then I plugged in a box fan. The power draw is just over 200 watts. This picture was taken near the beginning of the test.
I stopped the test after the inverter's low voltage alarm sounded. The test confirms that the cells deliver their advertised capacity.
Next, I charged the battery, let it rest overnight, then re-ran the same test. Instead of pictures, I took video. After reviewing the video, I plotted a graph of the voltage readings as the battery was discharged. The battery was at 14.94 volts at the start of the test. The inverter's low voltage alarm sounded at 10.92 volts. At that point, the battery had provided 41.3 amp hours worth of current or 534 watt hours worth of power. The total time from start to finish was 2 hours 55 minutes, 3 seconds.
This test is like the last one, except I used a 100w light bulb as a load. The battery was at 14.93 volts at the start of the test. The inverter's low voltage alarm sounded at 10.85 volts. At that point, the battery had provided 42.1 amp hours worth of current or 548 watt hours worth of power. The total time from start to finish was 4 hours 35 minutes, 14 seconds.
This test is like the last two, except I used both the box fan and a 100w light bulb as loads. The battery was at 14.94 volts at the start of the test. The inverter's low voltage alarm sounded at 11.04 volts. At that time, the battery had provided 39.8 amp hours worth of current or 506 watt hours worth of power. The total time from start to finish was 1 hours 37 minutes, 58 seconds.
Here is a graph of the 120w, 200w, and 320w tests, showing how the voltages compare.
Capacity test, 200w load, 15 volt charging voltage:
For this test I charged the battery with a reduced voltage of 15 volts, let the battery rest overnight, then ran the capacity test with a 200 watt load. Even with the reduced charging voltage, the battery still delivered 40.8 amp hours worth of capacity.
Capacity test, 200w load, 14.5 volt charging voltage:
This test is like the last one, except I used a charging voltage of 14.5 volts. With this charging voltage, the battery provided 38.4 amp hours of capacity.
Finally, I tested the resting voltages at 100%, 75%, 50%, 25% and 0% charge. For this test I used a charging voltage of 15.45 volts. After charging the battery, and leaving it sit overnight, the test started with the battery at 14.93v. After 10ah, with the battery at 75% capacity, I stopped the inverter and waited for the voltage to rise. The battery's resting voltage was at 13.70v. At 20ah / 50% the resting voltage was at 13.19v. At 30ah / 75% the resting voltage was at 12.86v. At 40ah or near 0% the resting voltage was at 12.40v. I also took a reading at 41ah. The resting voltage was 12.31v.
I mounted a cheap voltage / current meter into a small project box, then attached it, using double-sided sticky tape. The meter makes it easier to determine when the battery is reaching the end of the charging cycle.
Low Voltage Alarm
While my inverter has a low voltage alarm, it isn't any help when running other types of loads. Fortunately, stand-alone low-voltage alarms are cheap and they don't take up much room. This one is cost me less than $10. I set the alarm threshold to 11 volts, then mounted it using a small piece of double-sided tape.
To further protect the battery balancer, I cut out a piece of clear acrylic sheet. This type of plastic isn't very easy to drill. Standard drill bits don't work very well, on this type of plastic. I tried using a step bit, and it worked fine.
I used #6 sheet metal screws and nylon washers to attach the acrylic plastic sheet.
The battery box, that I'm using, has vents that could leak water if the battery is out in the rain. I used roof repair tape to cover them up.
These are two pictures that I took after the project was complete. I also weighed it. The total weight is 28.6 pounds.
Observations, Comments, Lessons learned
- Amazon orders take a long time to ship, especially if you use their free shipping. I recommend ordering parts from somewhere else, if you're in a hurry.
- Aluminum hardware is expensive. Shipping makes it even more expensive. It would be nice if DIY Central Shop offered aluminum hardware, as an option, even if they charge extra for it.
- M6 aluminum screws are really hard to find. I ended up ordering mine from China.
- I used two different kinds of ATO fuse holders. The ones that I ordered from Powerwerx cost less money and were of higher quality than the ones that I bought from the store.
- Here is a link to a US-based supplier that carries the same battery balancer that I used. I would have ordered from them, instead of China, but they were out of stock, at the time.
- Battery cell monitors are worth the money. I'd be overcharging the battery if I hadn't used one, in the beginning.
- LTO batteries don't require as high of a charging voltage as I originally thought. Even at 15.0 volts, one isn't giving up any significant amount of capacity.
- While I was able to make a battery charger out of a power supply, if I were to do it over, I'd order one of these instead.
|Lithium titanate battery cells, 40ah||6||DIY Central Shop|
|Busbars||5||DIY Central Shop|
|Aluminum Hex Nut, M12 x 1.75 mm Thread, pack of 5||3 packs||McMaster-Carr|
|Aluminum Washer for 1/2" Screw Size, 0.515" ID, 0.875" OD, pack of 10||2 packs||McMaster-Carr|
|Aluminum Split Lock Washer for 1/2" Screw Size, 0.512" ID, 0.869" OD, pack of 10||2 packs||McMaster-Carr|
|Noco Group 27 Snap-Top Battery Box||1||Amazon|
|Royal 1-1/4 x 6 x 10' White PVC Trim Board (Actual size 1" x 5-1/2" x 10')||1||Menards|
|Self-Adhesive Foam Camper Weather Strip||1||Menards|
|#12 x 2 1/2" flat head construction screws||1 pack||Menards|
|Blue Sea Systems Terminal Feed Through Connector 3/8" - Black||1||Amazon|
|Blue Sea Systems Terminal Feed Through Connector 3/8" - Red||1||Amazon|
|#10 x 3/4" pan head sheet metal screws||1 pack||Menards|
|Aluminum M6 screws, pack of 10||1 pack||Amazon|
|Automotive ATO In-Line Fuse Holder||7||Menards|
|10-12 gauge ring terminals, 1/4" stud||1 pack||Mills Fleet Farm|
|QNBBM 6S Active Battery Equalizer BMS Balancer||1||Ali Express / Green Store|
|#8 x 1 3/4" flat head construction screws||1 pack||Menards|
|#6 x 1/2" pan head sheet metal screws||1 pack||Menards|
|#6 nylon washers, 5 pack||3 packs||Menards|
|10-12 gauge ring terminals, 1/2" stud||1 pack||Ace Hardware|
|Marine Grade Tinned Copper Wire Lugs, 2 awg - 1/2" ring||2||Amazon|
|2 AWG Tinned Seamless Marine Lugs, 3/8" post||2||Tinned Marine Wire|
|2 AWG Tinned Marine Battery Cable, Red||3 ft||Tinned Marine Wire|
|2 AWG Tinned Marine Battery Cable, Black||2 ft||Tinned Marine Wire|
|Battery Terminal Boots 2 - 1 AWG, Set: Black and Red||1 pack||Tinned Marine Wire|
|16 AWG Tinned Marine Primary Wire - Black||1 roll||Tinned Marine Wire|
|16 AWG Tinned Marine Primary Wire - Red||1 roll||Tinned Marine Wire|
|18 AWG Tinned Hook-Up Wire||4 ft||Digikey|
|ISDT BattGo BG-8S Battery Cell Monitor||1||Amazon|
|Apex RC Products JST-XH 6S Balance Plug W/ 6" / 150mm Wire Lead - 5 Pack||1||Amazon|
|Apex RC Products JST-XH 6S 6" / 150mm Balance Plug Extension Lead - 5 Pack||1||Amazon|
|12-10 AWG Snap Connector 5 Female 5 Male, Yellow||2 packs||Home Depot|
|Insulated Bullet Splices #16-14 AWG||1 pack||Mills Fleet Farm|
|Littelfuse Blister Pack 10 Amp ATO Fuse||2 packs||O'Reilly Auto|
|Littelfuse Mega Fuse Holder||1||O'Reilly Auto|
|Littelfuse 150 Amp Mega Fuse||1||O'Reilly Auto|
|SMH SY175 Series 2 AWG 175A Breakaway DC Power Connector||1||Parts Express|
|Corrosion Proof Safety Boot for SB175 Amp Housings||1||Powerwerx|
|Tinned Copper Lug, 2 AWG, 3/8"||2||Powerwerx|
|Adhesive Lined Heatshrink 5/8", Red|
|Adhesive Lined Heatshrink 5/8", Black||1 pack||Home Depot|
|Cable Lug Tinned Copper Ring 2 AWG Gauge Stud Size 5/16 Inch||2||Genuine Dealz|
|ATC/ATO Inline Fuse Holder (Gauge: 10, Color: Red)||2||Powerwerx|
|Anderson Powerpole 15/30/45 Loose Piece Powerpole Colored Housing, Red||2||Powerwerx|
|Anderson Powerpole 15/30/45 Loose Piece Powerpole Colored Housing, Black||2||Powerwerx|
|Anderson Powerpole 45 Loose Piece 45 Amp Powerpole Contact||1 pack||Powerwerx|
|Roll Pins for 15, 30 & 45 Amp Powerpole Housings||1 pack||Powerwerx|
|10/2 AWG Duplex Tinned Marine Wire||4 ft||Tinned Marine Wire|
|Cable Tie Screw Mount||1 pack||Tinned Marine Wire|
|#12 x 3/4" pan head sheet metal screws||1 pack||Menards|
|12-10 Gauge Heat Shrink Ring Terminals, 3/8"||1 pack||Mills Fleet Farm|
|Butt Splice Heat Shrink Adhesive Lined Transparent 12-10 AWG Yellow Butt Connectors - 5 Pk||1 pack||Mills Fleet Farm|
|Battery Cable Boots||1 pack||Mills Fleet Farm|
|Mean Well RSP-750 Power Supply||1||Mouser Electronics|
|StreetWires UFX407S 4 AWG Power Cable Clear 7 ft||1||Parts Express|
|SMH SY175 Series 4 AWG 175A Breakaway DC Power Connector||1||Parts express|
|16/3 3' Indoor Power Supply Cord||1||Menards|
|Ring Terminals 4 AWG, #10||2||Digikey|
|Hirose DF11-14DS-2C connector housing||1||Digikey|
|Hirose Black 24 AWG Jumper Lead Socket to Socket Tin 4.00", 10 pack||1 pack||Digikey|
|Insulated Bullet Splices #22-18 AWG||1 pack||Mills Fleet Farm|
|12V to 5V 3A Step-Down Waterproof Miniature DC-DC Converter Power Supply Module||1||Amazon|
|Jianfa Rocker Switch||1||Amazon|
|Bayite DC 5-120V 100A Mini Digital Ammeter Gauge with Hall Effect Sensor||1||Amazon|
|Project Case, 2.4" x 1.4" 1"||1||Amazon|
|MPJ 35716MP 9 or 12V Battery Low Voltage Alarm Buzzer||1||eBay|
|AmeriLux 0.060" x 11" x 14" Non-Glare Acrylic Sheet||1||Menards|
|Quick Roof Ultra Bond White Self-Adhesive Roof Repair Tape 4" x 10' Roll||1||Menards|
Last Update: 12-28-2019
Copyright © 2001-2023, Brent Harris
All rights reserved.