DIY Campervan battery

Skitrot

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Nov 5, 2022
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Hi All,

I'm in the process of converting a van to a camper. Regarding the battery, I was thinking of making it myself mostly for the following reason. I work in a bike shop and we have a nearly endless supply of recent broken e-bike batteries. Mostly, it's the software that gives in, not the cells. I could collect about five 36v 10ah batteries a week if needed. Cells are LGDBMJ11865. Given that I've got this steady supply, I thought it stupid not to try and make my own battery.

The details. I would skip using an AC inverter and try to run everything on DC.
The total 24h consumption would be about 255Ah at 12v, but in my case it's easy to scale up the battery. I would not skimp on capacity.
The maximum current would be 40A. This includes a fridge, an electric kettle, two water pumps, lights, chargers, fans, audio etc.

How do I go about designing the pack itself? Do I build a massive 7S 24v pack and step down to 12v with a converter? Do I only need a 3S pack and use a converter to step it up to 12V? What about current rating? I plan on using a breaker box to distribute the power around the van. And the battery goes straight into the box? What device would go in between the board and the battery if any? Or will the correct converter with the correct current rating simply provide the right voltage and regulate the current both ways?

I would also like solar charging and an alternator to charge the battery while driving.
I know my way around electronics a bit and know how to read schematics and symbols, but electricity is somewhat new to me.

Thanks in advance, you battery wizards!
Rico
 
Build 7s pack buck converter down to 12V. 7s60p if the cells test out at 3000mAh each then multiply by the numbers of days you want to have in reserve. Plus a good BMS.
later floyd
 
I have a DIY 18650 battery in my trailer -> camper. We're in our 2nd year (10? camping trips) and it's holding up great.

Using RV (12v) heat-pads (https://www.amazon.com/gp/product/B07GW8ZX7X/ref=ppx_yo_dt_b_search_asin_title) for cold weather / then 1/2 foam (https://www.amazon.com/gp/product/B07G3J6294/ref=ppx_yo_dt_b_search_asin_title) underneath the packs. The rubber insulates the cells from the heat-pad electrically and is a shock-absorber. The packs are held in place with long twist-ties secured to the top of the cabinets (you can see in the pic). This is 14s88p.

I have MPP Solar 3048LV (inverter) so I use a 120vac -> 12vdc converter for 12v stuff but as @Floyd says - a 24vdc -> 12vdc converter will work just fine. You can't make an 18650 battery with proper 12v range so 24vdc (as you're planning) is advised.
1667685196955.png

It's dual axle trailer so it's similar to a car in terms of bounce/shake/stress on the battery. Shaking just hasn't been a problem.
 
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Good advice here so far.

7s (~24v) is the smart build for a usual camper. If any devices can be run at 24v (most fridges etc), that's easy for 12v, use a buck converter (gets $$$ for high amps). Using a cheap 2000w inverter for 120/240v when you need it is super simple to add later on.

I'm running a 7s120p (~7kwh) in mine. Have 2 x 40A Charge controllers. One from alternator stepped up to 48v (via a dual battery cut off) and the other from ~600w of solar on the roof. I also use a 25A battery charger to top up from 240v when at camp sites since even with all the juice coming from driving and sunlight, the battery can still get low after a few days of heavy esspresso and cooking. That's the main issue with campers. If the charge you're putting in is less than the use, that massive battery will keep dropping.

With the number of cells you've got available, you're in the enviable position to have options. Simplicity would be a massive 4s system only charged to 14.4v. You lose ~50% (3.6v instead of 4.1v max per cell), but could save $$ and time with all the faffing about boosting and dropping voltage. 3S slightly worse since you'd need to use a more complex charging system from the alternator and would Max out at 12.6v (12.3 if you're not stressing the cells).

What's your budget for all the non battery components?
 
Great thanks for all the replies! Some questions come into mind.

- What is exactly the advantage of making a 7S battery pack, regulating it to 24v with a BMS and afterwards stepping down to 12v. Why not build a 4s system and stepping down to 12v from there on by using a BMS? What is the downside to working like that. Or would you need a 4s BMS and a separate step down converter? Which also seems doable. Why build a 7s pack in the first place?

- In terms of vibration etc, I was thinking of reusing the housing the cells come in. I gather discarded recent e-bike batteries. Manufactureres would be spending too much time in getting these electronically back on track, so we discard them all. I assume they are also built to withstand some stress. In the housings I could easily build big 1S50P packs. I would assemble 7 of these when building a 7S pack for a total of 7S50P and get working. Afterwards, I'll plan on adding another 7S50P to the same BMS. Basically, I'd make several 3.7v high capacity packs ad wire them up as indivdual cells.
Or again, would it be easier to combine them in a 4S arrangement and what would be the downsides. I've got room inside the metal housing to add fuses for each group of 10 cells. A fuse for 10 cells with a 10a max discharge would mean a 100a fuse, or is it safer to use a lower value to cut out the load to the cells well before their max current rating. I was thinking of taking the wires from the cells to a pcb or breadboard and placing the fuses there. The housings can be screwed together on the sides, they have threaded inserts. That's how I would make one big pack out of them.
I'm attaching pictures of how the cells would be packed inside the housing, the housing itself and the two end that screw into place.

View attachment 1668854911815.jpegView attachment 1668855341599.jpegView attachment 1668855384900.jpeg

- Pricewise, I would like to stay below commercially available non li-ion batteries or thereabouts. I don't need li-ion per sé, it's just I have the cells quite easily available so why not. I think that if I make sure the electronics can handle bigger currents, I can easily add on extra cells later on? Or am I wrong in assuming this? So with a 50P system, I wouldn't be gaining much, but if I add another 50P afterwards I think I would.

Thanks!
 

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Great thanks for all the replies! Some questions come into mind.

- What is exactly the advantage of making a 7S battery pack, regulating it to 24v with a BMS and afterwards stepping down to 12v. Why not build a 4s system and stepping down to 12v from there on by using a BMS? What is the downside to working like that. Or would you need a 4s BMS and a separate step down converter? Which also seems doable. Why build a 7s pack in the first place?
Lithium-ion (e.g. most 18650 cells are lithium-ion) voltage range does not work as 3s or 4s for 12v applications. Lithium-ion has a working range of 3.4v low (there's no power left any lower) and 4.2v hi - e.g. really it's 4.15v max in a practical sense.

3s - 3 times the low and hi = 10.2v low and 12.45v hi. This voltage range is too low so you can't use a significant portion of the capacity.
4s - 4 times the low and hi = 13.6v low and 16.6v hi. This voltage range is too hi so again, you can't use a significant portion of the capacity.
Here's just one quick example of a typical 12v voltage range - https://surveillancetrailerrental.com/easy-12v-battery-voltage-chart/
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For lithium-ion, 7s / 24v - e.g. 23.8v low and 29.4v hi using the same numbers above - fits the voltage range of equipment almost perfectly. I know it doesn't make sense that 12v is no good and 24/48 is OK but it's true - the voltage ranges/equipment specs just shake out that way

If you want to go 12v then LifePo4 4s is a great match.
 
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What is exactly the advantage of making a 7S battery pack, regulating it to 24v with a BMS and afterwards stepping down to 12v. Why not build a 4s system and stepping down to 12v from there on by using a BMS? What is the downside to working like that. Or would you need a 4s BMS and a separate step down converter? Which also seems doable. Why build a 7s pack in the first place?
In addition to OffGridInTheCity's post, a BMS doesn't do any voltage regulation aside from making sure the battery voltage doesn't go too high, or too low. This is a set range. A Buck and/or Boost converter is what changes from one range to another, say for instance, a "24V" system to a "12V" system.
A BMS does just what its name states, it "manages" the battery. It doesn't alter the voltage output other than to disconnect until certain conditions.
 
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