Need help to size a BMS for either 2 x 12v 280Ah or 1 x 12v 560Ah

[Koontzy]

Need help to size a BMS for either 2 x 12v 280Ah or 1 x 12v 560Ah

I plan on charging it with 4 x 100watts 12v solar panels everyday and top it with a generator when there is no sun or too much power use. I plan to use these battery in my travel trailer or a small portable battery bank (in a wheeled tool chest or 2 small battery box)
I've found few 200-300-400Ah but I dont know if they are too big or too small. I've even bought by "mistake" one BMS rated for 300Ah 8S (when my first plan would have been 2 x 12v 280Ah)
my power consumption should be mainly TT stuff without any hair dryer or even air conditioner, but fews phone and computer charger and light (and water pump and furnace fan)
Any idea in which range I should look ? I'm totally at lost

 

[babybober]

You need to calculate your draw. Lots of websites out there where you can estimate what each item will draw power wise inside your camper. Will you be running an inverter to power AC voltage items like a microwave or coffeemaker? Those will be your big power draws. You need to concern yourself with max combined draw. If you run 2 280ah batteries in parallel with a bms on each the bms can be smaller because the load is split. Example, if you used a 120amp overkill solar bms on each bank of 280ah batteries you could draw 240amps of current at 12v at the same time. Now that would kill your batteries in about 2.5 hrs but... Bottom line unless you are attempting to use high power AC voltage devices the bms i just listed is overkill BUT not uncalled for. That setup would allow you to run a 2000w+ inverter in the future.

 

[Koontzy]

Actually I've calculated few time and end up with around 1500-2000watts. The inverter I'll use is the one already in my RV so it's hard to know. I don't know much about lithium cell so I don't know if I take two small 100Ah Bms and charge them in parallel I'll end up only charging them to 100A. Well I don't know this basic stuff ??

 

[babybober]

Ok 2000 watts is a lot of power in a camper for continual usage unless it includes Inverter items also. As a reference I am putting qty2 280ah 12v batteries in my 5th wheel and will run my Microwave, coffee maker, wife’s hair dryer, tv, etc. just about everything in there less electric heater and Air conditioner. Currently I am running 2 marine batteries that I probebly get 100ah total usable and that will run lights, radio, propane fridge, water pump, etc for about 5 days abut no AV voltage items. A BMS has a max charge and discharge amperage. That is the maximum throughput you can get in each direction at any given point not total. Think of it as a hose. The larger the hose the more that can pass through at any given point in time. The bigger the hose the more it costs.

 

[Koontzy]

ahah yeah I've oversized my usage a bit (it should be around 1500watts)
So I should size my BMS on what would be my amp usage instead of my whole battery A ?
Meaning I should be able to get away with a 100A BMS and this will charge my battery to 580A even if is a 100A BMS but only be longer to charge?

 

[schmism]

You need to find out what inverter is in your RV. You'll also need to factor in any DC loads that are run off the battery directly.

if its a 1500w inverter *15% (inefficient) = 1725w (draw at the battery) /12v = 143amps. + DC loads = 150a BMS

You mention 24v however you can only use a 24v battery setup if your inverter supports 24v and you run a DC-DC converter to power all the 12v items in the RV

 

[babybober]

ahah yeah I've oversized my usage a bit (it should be around 1500watts)
So I should size my BMS on what would be my amp usage instead of my whole battery A ?
Meaning I should be able to get away with a 100A BMS and this will charge my battery to 580A even if is a 100A BMS but only be longer to charge?

Correct. Your BMS should be based on constant usage need (read discharge rate on BMS) with ability to handle peaks of usage, NOT total usage over time that is capacity of your batteries ( in your case 540ah). If you never draw more than 1500watts at one point in time your BMS continuous discharge current needs to be 125amps (Watts=Volts x Amps). Remember that is NOT peak usage. Now for charging, yes you are understanding properly. BTW, 100A continuous charge is a lot in a camper. You don't indicate what country you are from but in the US the typical battery charger/converter in a camper is normally 45-55 amps. I upgraded mine to a 75 amp charger as a reference. Any larger than that and I could not plug it into a "typical" 120v/20A outlet.

 

[HRTKD]

That's a nice size battery bank. I chose to go with two 4s 280 Ah batteries, each with their own 120 amp Overkill Solar BMS.

As stated already, the BMS is rated for use, not battery Ah.

240 amp of capacity is a lot for the typical travel trailer, as long as it's understood that you're not likely to run the air conditioner. The microwave is doable as long as your inverter is capable.

 

[Koontzy]

Wow guys you are amazing !! (almost spent few $$$ for a 400-500A BMS)
I live in Canada so it should not be that far from US typical specs
Can I parallel my solar charge controller or generator on TWO BMS (so it would charge 2 smaller 12v 280a battery) ?
also about my inverter, I did not check how much watt it have, but most of my appliance will run on propane anyway
another question, I think I should use a breaker between my BMS and my inverter (just under the A of my BMS)
For the power consumption, I bough an old air conditioner yesterday but I don't plan to use it anyway, only fan
I'm pretty sur this question is stupid, but it would not hurt my truck alternator or the RV inverter if it try to recharge 2 lifePO4 battery through the towing wiring with lot of capacity when towing the trailer ? It would not be that much, 99% of the charging will be at home, through the solar panel or the generator
Now I just need to find proper battery enclosure, small heating pad with thermostat, few display gauge and stuff like that

 

[HRTKD]

A BMS is typically used on a single battery. A BMS does NOT charge the battery. It functions as a gatekeeper to ensure that the battery is not overcharged, over discharged as well as ensuring that the individual cells are protected the same way.

Yes, you should use a breaker between the BMS and the inverter.

For charging from the alternator, the general rule is that you should be using a DC-DC charger. This device will limit how much of a draw is made on the tow vehicle's alternator as well as providing a proper charge profile for the battery.

 

[Koontzy]

Yeah actually I'll use two bms with one battery each, but charged by only one power source for the two battery/bms assembly

 

[HRTKD]

Yeah actually I'll use two bms with one battery each, but charged by only one power source for the two battery/bms assembly

That's a normal setup. I have two solar charge controllers, and an AC-DC converter charging two LiFePO4 batteries.

The convenient way to make that happen is to use a set of common bus bars. All the loads, charging devices and the batteries connect at the common bus bars.

 

[Koontzy]

One last question guys.. Any downside of using two separate battery with their own bms in parallel. Ex2x150A bms for 300Ah output? Will these two bms play nice with each other in parallel?

 

[HRTKD]

One last question guys.. Any downside of using two separate battery with their own bms in parallel. Ex2x150A bms for 300Ah output? Will these two bms play nice with each other in parallel?

The main downside is the cost of an extra BMS and more cable. Otherwise, lots of positives for that approach.

 

[Bud Martin]

So the BMS you are using has the same current charging/discharging spec?

 

[Koontzy]

I plan on buying the same 100 or 150A. Actually I'll have two identical powerbank and when I'll need more power I'll parallel them

 

[HRTKD]

So the BMS you are using has the same current charging/discharging spec?

Yes, the Overkill Solar BMS spec for charge/discharge is 120 amps.

 

[Bud Martin]

I am just wondering which BMS the OP is going to buy.

 

[Koontzy]

Probably Daly bms with Bluetooth without screen 100A (around 100$ cad, maybe 75-80$ USD each)

 

[Bud Martin]

Probably Daly bms with Bluetooth without screen 100A (around 100$ cad, maybe 75-80$ USD each)

Then you should look the spec of DALY, more likely the charge current limit will be 50A. I have two dumb 100A DALY BMS:50A charge, 100A discharge spec.