DIY Lithium Packs, Proposal and Discussion

luthj

Engineer In Residence
In order to take advantage of the high rate charging with a 24 pack I would need several expensive DC-DC chargers. If I built a 24V pack it would be a single pack, not chained drop-ins. Charging the cells in 2X12V groups would not work with any single BMS that I know of.

The advantages of 24V are far outweighed by the cost and charging benefits of staying with 12V in my application.
 

john61ct

Adventurer
Has anyone considered or done a battery eject system..... as in mount your batteries on exterior outside wall.... or just under rear of camper box outside behind axle.... with a way to literally drop them if fire starts in the lipo's allowing you drive away from the batteries.
OMG, just use a safe chemistry!
 

john61ct

Adventurer
LiPo is not a chemistry as such, just a packaging technology.

And NCA vs LCO/LiCo vs LMN, NMC (NCM) all those LI chemistries are **much** more susceptible to thermal runaway than LiFePO4.

Only LFP and maybe LTO are IMO safe enough for House use in a mobile context.
 

john61ct

Adventurer
Do not use drop-ins for anything serious.

There are packaged system banks designed to be modular combine up to higher voltages or very high Ah capacities, but the starting point buy-in costs thousands, more than many spend on their whole vehicle.
 

john61ct

Adventurer
there are awesome battery charge equalizers that will use your 12v 100amp alternator to charge 12v batteries that make up a bank of 24v or 36v or..... (or equalize several banks of xvolt batteries wired to make up a 12v bank)
Links please.

By "equalizers", in this context do you mean balancing voltages?
 

luthj

Engineer In Residence
Looking at pack disconnect relays/contactors. I think the Gigavac GX12S or Blue Sea 7713 are good options.

THe GX12S is a dual power coil, which only uses 1W once closed. Low enough power for my application. It can handle several hundred amps continuous.

The 7713 uses only 13mA when closed, and has its own control circuitry, so it can be controlled without a separate driver circuit. It is rated for ~500A continuous, and has a manual override.

I have seen some 300A solid state BMS units for Ebikes and similar. I honestly don't think they would handle more than 150A. Though If I wanted to go all hardware hacking on them, I could replaces the FETS and heatsinks.

For reliability reasons, i prefer a mechanical contactor over the integrated solid state switches. I will give the external solid state switches a look later to see what the cost/packaging looks like. At least with the external/standalone contactor/switch units, I can bypass if a component fails. With the all-in-one designs, a single component failure may take the whole system down. I try to avoid this if possible, as failure for a full timer is a serious deal. No heat, no light, no food, no cooking. No Bueno.

Part of the reason the DIY BMS appeals to me, is that I can modify the code to control pack heater/cooler. I also would like an audible alarm/light during a caution event, to provide warning before the pack disconnects. This gives the operator (if present) some time to correct the issue before disconnect occurs.
 

john61ct

Adventurer
I agree with all that.

Personally I am skeptical of the idea of single monolithic "a BMS".

I see it as a collection of functions.

Some like balancing have nothing to do with protection, and usually done very poorly and not adjustable, often just stupidly designed.

Some need implementing at the cell level, some pack-level, some both, often redundancy is justified if the cells are worth a lot.

"a BMS" actually are **the cause** of bank failures very frequently, sometimes permanently rendering cells into scrap overnight. So build quality and careful thought around failure modes are important, but can cost way too much money.

But the same concerns apply to DIY, especially when noobs try to reinvent the wheel and **design** from scratch.

Being a noob myself especially to component level electronics, looking at the open-source designed projects out there like foxBMS is plenty ambitious.
 

luthj

Engineer In Residence
The BMS chips on some of the DIY type open source units eliminates some 90% of the components. These chips measure voltage and temperature, and have built in failsafes on the hardware level. The microcontroller attached just handles basic logic, and pulling the raw data from the BMS chip. Which is easily understood at the coding level, and is quite reliable.
 

john61ct

Adventurer
As long as the setpoints are adjustable, that's one area I've been disappointed looking at these type chip datasheets
 

luthj

Engineer In Residence
The LTC6804 and similar can be fully configured over the data bus. Some of the chips have no logic of their own, and relay on external control (or at least configuration.

I agree though, many of the cheaper chips are designed for LCo chemistries, and are supposed to be "plug and play" for the makers of powered toys, ebikes, etc.
 

shade

Well-known member
The LTC6804 and similar can be fully configured over the data bus. Some of the chips have no logic of their own, and relay on external control (or at least configuration.

I agree though, many of the cheaper chips are designed for LCo chemistries, and are supposed to be "plug and play" for the makers of powered toys, ebikes, etc.
Now you have me curious about the internals of the Victron BMS. I'll have to see if I can open it and take a peek at the chips.
 

luthj

Engineer In Residence
I would be curious if you can read the part numbers. I am certain they use off the shelf parts. Like any design, the details make or break it.

I bet they are using a standalone balance chip inside the battery itself (with fets and resistors). I would be curious if it uses bi-directional balancing, or if its simple shunt style.
 

dreadlocks

Well-known member
dont listen to him.. void, void, void.

seriously tho, warranty void if removed stickers are unenforceable, as long as you can crack it open w/out physically damaging it.. you'll be fine.. void, void, void.. you kno u wanna :p
 
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