Low cost DIY Lithium camper batteries.

[rruff]

That's an odd thing to want to do. The battery characteristics are different for starting and long slow draining. Plus you would need some serious fat wire running to your truck starter. What is the reason for your idea?

Simplicity and weight savings. Being able to use the alternator for house battery charging without isolation, and getting rid of a starting battery that I don't really need.

AGM or FLA deep cycle batteries will easily start a vehicle if they are big enough, and they only need to be a little larger than the starting battery to account for slightly higher internal resistance. I was planning on ~3x the size so plenty of CCA.

I'd need big wires, but typical starter wires will work. I was planning to put the batteries behind the front seat of the cab.

But... that was AGMs. Thinking about it more, if I use LiFePO4 for the house I can probably forget about alternator charging, because they are not finicky about needing to be topped off the way AGMs are. Just need to make sure my panels and batteries are sized in excess of what I'll use.

 

[rruff]

If you used it for starting you could see draws more like 100A, so you would want to use heavier gauge bus wire between the cells, and much heavier wire between the packs. You would also want a BMS/protection circuit rated for a higher current.

Yes, I looked at BMS options and the high power ones are expensive! Plus I'd need a voltage step up on the alternator for it to charge lithium. Thinking it makes the most sense to have them totally separate and just use solar for the house battery.

I've been reading up on connecting the cells and a lot of people are negative about soldering. They say it damages the cells. Spot welders are kinda pricey though. Decent ones are ~$200 with a pen for large packs. Soldering definitely heats up the cell more, but I don't know if anyone has measured if it is enough to matter at all (by life cycling the battery).

 

[PaulT]

There are many lithium based portable starting packs that contain a 8 to 15 amp lithium battery and are able to start engines. You could look into the technology/circuits that make that happen to see if the technology could be adapted to accomplish what you want.

Having a small starting system in the engine compartment without needing massive wires from the deep cycle camper batteries would be attractive and the starting battery would be charged by the camper system.

I have no idea how practical it could be to invent the system to accomplish what you want, but it seems possible.

Paul

 

[rando]

Yes, I looked at BMS options and the high power ones are expensive! Plus I'd need a voltage step up on the alternator for it to charge lithium. Thinking it makes the most sense to have them totally separate and just use solar for the house battery.

I've been reading up on connecting the cells and a lot of people are negative about soldering. They say it damages the cells. Spot welders are kinda pricey though. Decent ones are ~$200 with a pen for large packs. Soldering definitely heats up the cell more, but I don't know if anyone has measured if it is enough to matter at all (by life cycling the battery).

I think using separate batteries is still a better idea, but you could at least use a much smaller starting battery as you have the ability to switch in the house battery to help out the starting battery when needed. This reduces the need to oversize everything in your house battery and gives you some redundancy.

I think the discussion of soldering batteries vs welding batteries is a bit of internet lore. At least with these larger 32650 cells and a 140W soldering iron I could barely feel that cells warmed up at all while soldering - a few degrees C at most. The key is to use an aggressive flux, and a high wattage soldering iron/gun so that you only have to apply heat for a few seconds to each cell. The reason professional pack assemblers use spot welders is that it is much faster and more consistent even with low skilled workers, but the electrical connection is generally not as good as the actual bond area is tiny.

 

[cwdtmmrs]

rando, very cool project! I have a couple of questions. I do not have solar and only charge from my alternator. Also my battery is mounted in the truck bed in front of the passenger wheel well and is more exposed to the elements. What changes would you make in construction, if any, to accommodate these factors?

Thanks,
cwd

 

[rando]

For your application I think you would want some sort of weather proof case - something like a battery box, a plastic ammo can or cheapo pelican case knock off would work well. Pick something big enough to hold the number of cells you want, then start playing some tetris to figure out the cell arrangement. Using the smaller cylindrical cells means you can arrange them to fit just about any shape.

As for alternator charging, it all depends on your alternator voltage. In my case, my alternator is ECU controlled and quickly drops down to ~13.5V, when combined with my skinnier wires to the battery doesn't provide much charging current. If your alternator stays up at 13.9 - 14.2V you should charge just fine until the BMS shuts off the batteries. If you have heavy gauge wires running back there, you may want to check that you don't pull too much current. These batteries will happily charge at 1C, so 150A draw! You would also need to size your BMS appropriately.

Another, more expensive but more elegant option is a DC-DC charger, which would both boost the voltage and limit the current. You would want one that is adjustable for LiFePO4 voltages.

 

[cwdtmmrs]

Rando, thank you or the advice. I do have heavy gauge wires from the truck I have added 4 gauge wires and left the original 10 gage wires in place as well. My alternator is not ECU controlled and does put out 13.9-14.2V/14.4V. It is 120 amp and can produce 60 amps at idle. I don't use much power normally and my 55AH Optima Yellow Top battery has served me just fine, but it will be 10 years old this Spring and I probably should replace it soon. Your DIY lithium pack looks like a fun project and could triple or quadruple my capacity for the same weight.

Do you have suggestions as to which DC-DC charger and BMS to use? Would you use larger gauge wire for the internal cells? I would also need a clean, sturdy way to hook my existing battery cables to the via battery posts or lugs.

Thank you so much for all of your information!

cwd

 

[Vic Harder]

cwd,

With the 4 gauge wire and 14.4v output, assuming a healthy 40A charging current, you will see 13.6v or so at the batteries. You should not need a dc-dc converter, just the BMS.

 

[rando]

It is definitely a fun and satisfying project.

I have not used a dedicated DC-DC charger, so hopefully someone else can chime in with suggestions.

One idea I have had is to use on of the Victron MPPT solar chargers that I seem to always be talking about as a DC-DC charger. The way to do this would be to buy the MPPT charge controller and buy a 12v - 24V converter and attach the output to the solar panel input of MPPT controller.

This would have a couple of advantages:
1. If you get the SmartSolar version, you get bluetooth control and monitoring of the charger ,so you can adjust your set points and watch your battery charge from your phone/tablet in the cab of the truck as you drive. If your battery is not easy to get to, this could be important.
2. If you hook up the camper to the load terminals of the charge controller you can also watch your consumption on your device, so it operates as a more detailed BMV/Trimetic. The cool thing is that it gives you more than just the net charge/consumption, it gives you separate measurements of charge and discharge.
3. You get a built in low voltage / high voltage disconnect which will protect your batteries as a backup to the BMS.
4. If you ever do add solar, you are set up and ready to go - you could even have a switch to switch between alternator charging and solar charging using the same controller.
5. I think the cost would be about the same as a similar quality DC-DC charger.

I did rewire my camper so that the loads go through the load terminals on my new SmartSolar 100/20 for the reasons listed above.

 

[cwdtmmrs]

Vic, I am getting closer to 13.8-14 volts at the camper battery.

Rando, Thank you for the suggestions. You are correct in that I cannot get to the battery once the camper is mounted on the truck. I though about modifying the inside of the camper with a hinged lid, but it has never been an issue so not a priority. I'M not smart enough to use a smart phone! I'm not much of a tech type guy, I don't even own a TV and still can't keep my clock radio from flashing 12:00! I don't foresee ever adding solar unless it is a very small portable just to keep up with the computer fridge fans when I am backpacking for more than a couple of days.

cwd

 

[Vic Harder]

Vic, I am getting closer to 13.8-14 volts at the camper battery.

cwd

You will see that for sure, once the current flow drops. The more/faster current flow, the lower the voltage.

 

[cwdtmmrs]

Rando, using the measurements you posted for the 30 pack cell of 7"x8"x2.5", standing the packs on edge would yield a battery about 7"Wx8"Hx13'L, which is almost the same as a group 31 battery. Does that sound right?

Thanks,
cd

 

[rando]

Your dimensions sound about right - you do need a little space for the wiring and the protection circuit - but not much and you would have enough extra length in the 13" dimension to accommodate this.

One other thing to note - if I were to do this again I wouldn't bother with the 3D printed frames to hold the batteries. They were time consuming to print, and didn't add much to the finished product except making the 4 individual packs very rectangular and wasting space between each cell. Doing it again, I would glue the cells together, and then strap each pack together with strapping tape. I would assembly the whole battery in a plastic case (pelican case or the like as we discussed before) with closed cell foam padding to fill in the gaps. The overall battery would be smaller and easier to handle and more rugged.

 

[cwdtmmrs]

Thanks! Good info on the gluing together because I my next question was going to be about the program for the 3D printer!

With my small requirement of AH/day, this battery would last me almost a week.

I remember reading about limited or no charging on the LiFePo batteries in below freezing temps though. Any thoughts?

cwd

 

[rando]

The low temperature charging can be an issue, depending on your climate and configuration.

I was originally quite concerned about it, living in CO and using the camper year around, but it has turned out not to be an issue for us. Charging below freezing is OK, as long as the charge rate is low compared to the battery capacity - Victron suggests a maximum of 0.05C. We primarily charge with solar, and in the winter my 240W flat mounted panels only produce a peak of about 7-8A anyway, so we are around 0.05C of our 150Ah battery. To make sure, I set my MPPT charge controller to limit the charge current to 9A when the camper is just sitting in the driveway (easy to do in the app). When we are using the camper, we keep the inside > 0C to keep our water from freezing, so I remove the current limit.

In other cases this could be more of an issue. Some BMS will prevent charging (but not discharge) below 0C. The one I used doesn't have this limit, but I did come up with a simple modification to add it, which I never ended up implementing. You could also add a limit to your battery isolator - if you have the blue seas SI-ACR, place a 0C thermostat between the isolate terminal and 12v and it will prevent the batteries charging below 0C.

 

[rruff]

I believe that limited charging at below freezing temperatures is the case.

"Li ion can be fast charged from 5°C to 45°C (41 to 113°F). Below 5°C, the charge current should be reduced, and no charging is permitted at freezing temperatures because of the reduced diffusion rates on the anode. During charge, the internal cell resistance causes a slight temperature rise that compensates for some of the cold. The internal resistance of all batteries rises when cold, prolonging charge times noticeably.

Many battery users are unaware that consumer-grade lithium-ion batteries cannot be charged below 0°C (32°F). Although the pack appears to be charging normally, plating of metallic lithium can occur on the anode during a sub-freezing charge. This is permanent and cannot be removed with cycling. Batteries with lithium plating are more vulnerable to failure if exposed to vibration or other stressful conditions. Advanced chargers (Cadex) prevent charging Li-ion below freezing.

Advancements are being made to charge Li-ion below freezing temperatures. Charging is indeed possible with most lithium-ion cells but only at very low currents. According to research papers, the allowable charge rate at –30°C (–22°F) is 0.02C. At this low current, the charge time would stretch to over 50 hours, a time that is deemed impractical. There are, however, specialty Li-ions that can charge down to –10°C (14°F) at a reduced rate."

http://batteryuniversity.com/learn/article/charging_at_high_and_low_temperatures

 

[rruff]

This is probably a dumb question, but would it be ok to remove the paper wrapper before gluing the cells together? It looks like there is a plastic insulation ring on the + end. And the rest of the casing is -, correct?

 

[rando]

As long as you can still isolate the +ve terminal, I think this would be fine. I don't remember if the plastic insulating ring covers the whole top of the battery or not. You just need to be careful handling them as you are right that the entire case is -ve. Another option would be to switch out the paper wrapper for heat shrink tubing which would make them easier to handle and glue.

If you do want to use battery frames, they are now available on ebay:
https://www.ebay.com/itm/10pcs-lot-32650-3x-Battery-Plastic-Holder-Bracket-Cell-Safety-Anti-Vibration/142557200401?epid=2242937505&hash=item2131124011:g:I9IAAOSwzQFZ8wGB

 

[rruff]

I ran across some discussion of low charge rates not being good for LiFEPO4.
https://www.eevblog.com/forum/beginners/how-charging-lithium-(lifepo4)-batteries-works-in-the-face-of-changing-current/


"Lithium based rechargeable batteries can be damaged (over time) by slow charging currents. This has to do with dendrite growth on the edges of the plates. You do NOT want to trickle-charge these batteries. The only exception I know of is Lithium Titanium Oxide [LTO] chemistry, which is very tolerant to slow charging (but not trickle charging). LTO batteries are still very new and they do not have the nice flat discharge curve that LiFeP04 batteries have. LTO batteries are currently used in very high reliability applications (like medical implants).

If you have a source of power that is unpredictable (such as solar panels, or some other energy harvesting device), it is best to store up energy in a capacitor, and then dump this charge into the battery at a C/2 rate, then let the cycle repeat. The repetition cycle can be as rapid as your source will allow. This type of charge profile will minimize dendrite growth and maximize the lifetime of your batteries."

I was thinking of a 150ah battery so I'd have plenty of room for expansion later. Initially I'd just have a 160W panel. At 10a charge I'd have a .07C charge rate and it wouldn't get much higher than that. A lot of the time it will be lower, with the sun at an angle and cloudy days.

Thoughts?

 

[enelson]

I've been charging our 160AH LiFePo batteries at a low C current for around 4 years now and I can't tell any loss in performance of the batteries. I wouldn't worry about charging from solar cells or a solar/truck combination. Charging at C/2 is a lot of current on large batteries. I probably won't get 20 years out of the batteries and will be happy any use after 10 years.