Need a little help designing lifepo4 system with alternator charging

baltik

New member
Looking to swap out my 120ah Trojan flooded setup to a lifepo4 pack on my VW westy syncro. I want to use 180-200ah prismatic battery cells, most likely CALB cells. I would like to use the Electrodacus sbms system given the built in monitoring as well as customization. From what I understand I need a way to provide constant current current from the alternator to the sbms (approx 30A seems to be ideal) The sterling dc to dc chargers are a huge form factor not to mention cost and complexity. Redarc bcdc series would also do the trick but seem very redundant for what I want to achieve. Would love to know how others are approaching this. I know most people would go solar in this case but we tend to camp in the redwoods and don’t get enough direct sunlight...
 

dreadlocks

Well-known member
200ah of lithium seems like a ton of power for a lil Westy, what are you running in there? a 100aH of lithium is going to be over 2x as much capacity as you currently have and you think you need 4x as much as you got now?

I've thought about how i'd do my westy, I'd still keep a lead starter battery and run all the appliances off a 100aH LiPo.. with what @john61ct suggested as primary power source.

How are you recharging? Driving it? because idling an engine for hours on end is not good on it.. if solar is not going to be optimal, a small genset w/AC charger would be better than using your engine.. I learned that lesson the hard way, and alternators dont put out full power at idle either.
 

baltik

New member
Well we are inefficient users of the truck fridge (kids going in and out constantly) and want to be able to put in warm beers to chill them. Right now our deep cycle Trojan is done after 2 days or so. We also recently added a propex and want to be able to recharge MacBooks (usb-c) so for the westy form factor we are heavy users. I think 150ah would be enough but I am finding 180 cells to be similar size and price so why not... we rarely stay at a place for more than 3 nights so the battery needs to last 3.5-4 days and then charge when we drive to the next destination. We would not be idling to charge.
 

dreadlocks

Well-known member
hey put as much as you can in, but my 100ah runs my engel 90qt with a family of 4, a handful of USB-C gadgets (macbooks, nintendos, pixelbook) for 4 days no problemo, if we just burn energy and let lights stay on lots and keep restocking fridge, I usually run generator on day 3, and not for very long at all since I dont have to go all the way back to 100%.. I dont have my propane furnace in yet decided to get but I got a wave heater right now that dont use any electricity... the math from lead to lithium is not straight forward, there is so little losses putting power in and out that you get all these capacity bonuses that you didnt expect.. for example with lead, 1A for 10H is alot less consumption than 10A for 1H.. that goes away with lithium both loads are basically equal to eachother, so when your lights are on, all ur stuff is charging and your fridge is running your not just ramping up your losses like you were on lead.

just dont be surprised that you end up having like a week's worth of storage capacity, good thing is lithium prefers to be at a partial charge.. opposite again of lead that you wouldent ever want to let it dwell that long without being topped back off, since they prefer to be at full charge.

also consider is your next destination a full charge away? I'd need about 6h on the road to charge mine all the way back up if I do this system again the Westy and charge at about ~30-40A, if I had twice the battery I'd need twice the amps or it'd take twice as long.. if every 3-4 days you only drive 2-3h and your on a couple week vacation touring parks then your still going to need another charge source.. at least a good AC charger so you can top off at opportunistic locations along the way
 
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Verkstad

Raggarkung
at least a good AC charger so you can top off at opportunistic locations along the way
Consider connecting that charger with a SAE J1772 inlet so you can utilise electric car charge power also. Its surprising how abundant those are nowdays, and often ’free’. Power paid for by somebody else...
Mind you, its minor extra geekery to it than simply fitting the inlet, but its well within capacity of any legitimate ’ExPo Guy...
 
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baltik

New member
DCDC Sterling, BB series. Custom profiles, can front to any 12V charge source
The sterling line was definitely my starting point, and remains a viable option. At the end of the day all I need is a constant current converter that can deliver a steady 30 amps and has a remote off trigger so that the sbms can shut it off when full charge has been achieved. It just seems a shame to pay in money and size for all the extras in the sterling that are completely unnecessary for my use...
 

dreadlocks

Well-known member
30A seems a little low to me, I've got a 45A AC charger on mine and I'm planning on about 50A of Solar (Optimal) next season.. if your really shooting for 200AH, I think 60A would be a better starting point.. part of the charm of lithium is it'll basically take as much as you can give it til its full, none of that lead crap where 60% of the time needed is a fractional absorption charge.

@Verkstad the idea of a Westy parked in EV spot makes me happy, I'm sooo gonna do that for my 75 w/diesel engine in it.. mwhaha.. but yeah if this guy camps redwoods, he's obviously noracal and those charging stations are at like every grocery store on west coast it seems..

You can also charge up if you stop and visit family along the way, or if your really friggin pressed for a charge you can get an electrical spot for a night.. KOA camps suck but they are just about everywhere.. Ive done a many extended car camping trips and we hadda splurge and find electricity every once and a while to recharge all the batteries for cameras and stuff.
 
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jonyjoe101

Adventurer
I have a 220ah lifepo4 battery pack, on mine all I use is a 6 dollar overvoltage relay that triggers a larger 30 amp relay when the battery reaches the cutoff voltage. Its all I ever needed. The 6 dollar relay will do everything the other units will do.
On my battery pack I have active balancers (balance at up to 10 amps per cell) this way I can use the battery overall voltage to control when to disconnect the charging.
The battery also has a bms, but its best not to rely on that. Thats just for worst case scenario in case overvoltage relay fails.

In this diagram, replace the solar panel with the alternator, and instead of a 30 amp relay use a larger solenoid, the overvoltage relay has a built-in 10 amp relay which can trigger a much larger relay. The overvoltage relay is fully programmable, extremely reliable. As you can see the overvoltage relay is connected directly to battery and has the true battery voltage directly from the terminals.
diagram a.jpg

This is a picture of the 4s balancers, cost is about 100 dollars (available at electricarpartscompany). They balance all the time during charging/discharging. When fast charging at high amps all lithiums start to drift out of balance which usually triggers the bms to stop charging. You never get a full charge when that happens. Using these balancers I just rely on the overvoltage relay to stop charging when the battery voltage (lifepo4) reaches 14.4 volts. I would definitely recommend them for your large battery pack which you plan to charge at high amps.
active balancers.jpg

I would probably look at the chargery bms16 (cost about 100 on ebay) before I consider the Electrodacus sbms , the chargery monitors balancing and can trigger relays to shutoff charging if it goes out of balance. My 6 dollar relay works for me but the chargery is when you need even more precision.
 

john61ct

Adventurer
The sterling line was definitely my starting point, and remains a viable option. At the end of the day all I need is a constant current converter that can deliver a steady 30 amps and has a remote off trigger so that the sbms can shut it off when full charge has been achieved. It just seems a shame to pay in money and size for all the extras in the sterling that are completely unnecessary for my use...
Nope

Pretty common for a good charger to cost more than the bank, and will help you double lifespan of the bank if you use it right.

All the other inputs can be cheap since only the DCDC needs to be adjustable.

Bargain at twice the price, nothing else I know comes close.
 

baltik

New member
Lots of good info packed in your post jonyjoe. How are you measuring SOC in your setup? I see a digital display inyour balancer picture can you tell me what you are using?

Also I still need a way to limit current from the alternator- otherwise the batteries will take the max amount and fry the alternator
 

luthj

Engineer In Residence
Whats your alternator voltage? If its less than 14V, you can likely charge direct via a relay. What is the charge current limit on your system? With prismatic cells its probably quite high. Higher than your alternator can supply. With big wire you could max out the alternator. Not a big deal, as 50-70A is probably well within the packs limits.
 

baltik

New member
My understanding is that running an alternator flat out will result in overheating and a very short life. Also wouldn’t the starting flooded battery not appreciate 60+ amps?
 

luthj

Engineer In Residence
My understanding is that running an alternator flat out will result in overheating and a very short life. Also wouldn’t the starting flooded battery not appreciate 60+ amps?
Most alternators are fine with their rated output. Most will have thermal de-rate as well. If you are concerned, you can size your charging circuit wire to drop about half a volt at 30-50A. This will effectively limit the charge rate to the lithium pack.

You misunderstand how the current will be flowing. The alternator is a voltage source with a current limit. If the voltage remains the same (as it will) the starter battery will accept the same current it always does. This is regardless of what the lithium pack is charging at.
 
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