100a BMS x 2 = 200a BMS?

[jaschembra]

I have 2 - new 100ah LiTime batteries set in parallel. They each have a 100 BMS. Do I now have a 200 BMS? When purchasing them, I did not understand BMS, and with 100 BMS, our RV microwave will not run - we do not have our inverter yet, but LiTime tells me that we now have a 200 BMS since they are set in parallel. Just wanting to get educated. I read where if the batteries get unbalanced we may not then have a 200 BMS - how likely is that to happen? Thank you

 

[Rednecktek]

Yes, each battery does half the work so with two of them you'll have 200a of max draw.

 

[Mattb4]

Think of the BMS like a fuse or circuit breaker. You can pull the BMS rated amps from your battery. With 2 batteries you don't have a 200a BMS but you do have the ability to add the amps from each battery in parallel. This rating does not necessarily correspond to your batteries capacity anymore than running a small 2000w generator through a 200a breaker would suddenly give it 200a worth of power. Your batteries capacity is rated in amp-hours. In parallel you can add the amp-hour, in series you add the voltage.

In the case where one paralleled battery's capacity does not equal the other one each share the current base on the percent of capacity difference. Say you have a 100ah battery paralleled with 200ah battery. The load is 30a. The 100ah battery will supply 10a for every 20a of the 200ah battery.

 

[jaschembra]

Think of the BMS like a fuse or circuit breaker. You can pull the BMS rated amps from your battery. With 2 batteries you don't have a 200a BMS but you do have the ability to add the amps from each battery in parallel. This rating does not necessarily correspond to your batteries capacity anymore than running a small 2000w generator through a 200a breaker would suddenly give it 200a worth of power. Your batteries capacity is rated in amp-hours. In parallel you can add the amp-hour, in series you add the voltage.

In the case where one paralleled battery's capacity does not equal the other one each share the current base on the percent of capacity difference. Say you have a 100ah battery paralleled with 200ah battery. The load is 30a. The 100ah battery will supply 10a for every 20a of the 200ah battery.

Thanks Matt. I was concerned with - would the 100a BMS shut me down due to its safety features even though my capacity is now 200amps.

 

[Rednecktek]

Thanks Matt. I was concerned with - would the 100a BMS shut me down due to its safety features even though my capacity is now 200amps.

Only if you drew over the 200a available from the batteries, give or take.

 

[Mattb4]

Thanks Matt. I was concerned with - would the 100a BMS shut me down due to its safety features even though my capacity is now 200amps.

Be careful in thinking capacity since that is a different thing. For instance you can be shutdown at 100a draw simply because one of your batteries is not working or dead. But up to it you may not even know that has happened.

 

[Supervstech]

Think of the BMS like a fuse or circuit breaker. You can pull the BMS rated amps from your battery. With 2 batteries you don't have a 200a BMS but you do have the ability to add the amps from each battery in parallel. This rating does not necessarily correspond to your batteries capacity anymore than running a small 2000w generator through a 200a breaker would suddenly give it 200a worth of power. Your batteries capacity is rated in amp-hours. In parallel you can add the amp-hour, in series you add the voltage.

In the case where one paralleled battery's capacity does not equal the other one each share the current base on the percent of capacity difference. Say you have a 100ah battery paralleled with 200ah battery. The load is 30a. The 100ah battery will supply 10a for every 20a of the 200ah battery.

Not exactly.
The only way a 200Ah and 100Ah set would balance that way is if it was wired with intelligence.
No, most likely, a 100Ah and 200Ah in parallel would only partially increase the capacity.
The load should be equally shared between the batteries, so, at 200A load, 100 is on the 100Ah and 100 on the 200Ah, at 205A, the 100A battery now has 102.5A and will start over amp limiting...

Yes, the 200Ah battery has a better cabling, so it may handle some of the charge easier than the 100Ah, but no it will not take 50% more like you think it should.

However, in a large drain load, the 200Ah battery will not deplete as quickly as the 100Ah will.

 

[Mattb4]

Not exactly.
The only way a 200Ah and 100Ah set would balance that way is if it was wired with intelligence.
No, most likely, a 100Ah and 200Ah in parallel would only partially increase the capacity.
The load should be equally shared between the batteries, so, at 200A load, 100 is on the 100Ah and 100 on the 200Ah, at 205A, the 100A battery now has 102.5A and will start over amp limiting...

Yes, the 200Ah battery has a better cabling, so it may handle some of the charge easier than the 100Ah, but no it will not take 50% more like you think it should.

However, in a large drain load, the 200Ah battery will not deplete as quickly as the 100Ah will.

No amperage is not shared equally between batteries of different capacity. And yes a 100ah battery in parallel with a 200ah will increase capacity to 300ah. It is what the math of parallel sources tells you it does. In parallel amp hours add up.

For instance in my example of a 30 amp load on paralleled 300ah worth of batteries. 300ah/30a=10 hours. For each battery to operate for 10 hours the 100ah/10h = 10 amps and the 200ah/10hrs=20a This is straightforward before you toss in differences in cabling.

Yes in the real world of unequal size components there is going to be differences but the basics don't change.

Think of it like two water tanks of different sizes connected together. If you have a common drain the level drops in each tank equally but the amount each tank contributes to the flow is proportional to their size.

Edit: One other thing. Rating of battery capacity is not perfect. No two batteries will have the same exact capacity. In essence you are always paralleling batteries of different capacity. Could be an actual 105ah with a 95ah even though both are nominal 100ah. So their charging and discharging proportion is never quite equal amperage.

 

[jaschembra]

So, going back to my original question and I’ll use my exact example for needing to know - we have 2 lithium batteries (12V 100ah) and each one has a 100amp BMS - since these 2 batteries are balanced and tied together in parallel - my BMS shouldn’t stop things from running unless I pull over 200 amps - my RV microwave should run if it is pulling 110 amps? The 1000w microwave’s continuous input is 1550 watts so 1550/12v x .85 = 109.8. There would be room for surge at startup. Just making sure, otherwise I may try to exchange the batteries for 1 230ah that had a 200 BMS. Thanks

 

[Mattb4]

So, going back to my original question and I’ll use my exact example for needing to know - we have 2 lithium batteries (12V 100ah) and each one has a 100amp BMS - since these 2 batteries are balanced and tied together in parallel - my BMS shouldn’t stop things from running unless I pull over 200 amps - my RV microwave should run if it is pulling 110 amps? The 1000w microwave’s continuous input is 1550 watts so 1550/12v x .85 = 109.8. There would be room for surge at startup. Just making sure, otherwise I may try to exchange the batteries for 1 230ah that had a 200 BMS. Thanks

Yes if the batteries are functioning properly and have the capacity they will pick up their share of the load up to their BMS rating. Remember it is 2 BMS's. Each maxes out at 100a. None can exceed 100a in getting to 200a total.

 

[jaschembra]

Each maxes out at 100a. None can exceed 100a in getting to 200a total

But an inverter wouldn't get shut down due to "100a BMS", it would basically think it was pulling from 200a BMS (all things considered equal and at full capacity), correct? The inverter company told me yesterday that it would shut down but I don't know if they truly saw that I said there were 2 in parallel. I don't want to have to carry a counter top smaller microwave if I can use the residential one in my camper. Thank you Matt

 

[Mattb4]

But an inverter wouldn't get shut down due to "100a BMS", it would basically think it was pulling from 200a BMS (all things considered equal and at full capacity), correct? The inverter company told me yesterday that it would shut down but I don't know if they truly saw that I said there were 2 in parallel. I don't want to have to carry a counter top smaller microwave if I can use the residential one in my camper. Thank you Matt

An inverter would only get shut down if the voltage being fed to it from the batteries drops below minimum. So what they told you is incorrect. This can happen when CS folks read from scripts and are not technically trained individuals. What your thoughts were are correct.

 

[Supervstech]

No amperage is not shared equally between batteries of different capacity. And yes a 100ah battery in parallel with a 200ah will increase capacity to 300ah. It is what the math of parallel sources tells you it does. In parallel amp hours add up.

For instance in my example of a 30 amp load on paralleled 300ah worth of batteries. 300ah/30a=10 hours. For each battery to operate for 10 hours the 100ah/10h = 10 amps and the 200ah/10hrs=20a This is straightforward before you toss in differences in cabling.

Yes in the real world of unequal size components there is going to be differences but the basics don't change.

Think of it like two water tanks of different sizes connected together. If you have a common drain the level drops in each tank equally but the amount each tank contributes to the flow is proportional to their size.

Edit: One other thing. Rating of battery capacity is not perfect. No two batteries will have the same exact capacity. In essence you are always paralleling batteries of different capacity. Could be an actual 105ah with a 95ah even though both are nominal 100ah. So their charging and discharging proportion is never quite equal amperage.

Dont confuse Ah with Ampdraw.
Yes, the bank will provide 300Ah of capacity, but, No you would not be able to pull a 300A draw on the bank.

 

[Gueromeister]

So how is it better to have one 12v 200ah with a 200 BMS vs how is it better to have 2 12v 100ah with 100 BMS vs how is one 12v 200ah with a 100BMS better ? Pros & cons ….

 

[jaschembra]

So how is it better to have one 12v 200ah with a 200 BMS vs how is it better to have 2 12v 100ah with 100 BMS vs how is one 12v 200ah with a 100BMS better ? Pros & cons ….

I am not knowledgeable enough to answer but I will say this - we returned our 2 12v 100Ah batteries with the 100BMS (within a 30 day window) and replaced them with 2 12v 230Ah batteries which each have a 200BMS - We decided we wanted a "bigger gas tank". Of course then, we went down a rabbit hole with our future.... increase in inverter, fuses, cables, etc.

 

[Gueromeister]

I am not knowledgeable enough to answer but I will say this - we returned our 2 12v 100Ah batteries with the 100BMS (within a 30 day window) and replaced them with 2 12v 230Ah batteries which each have a 200BMS - We decided we wanted a "bigger gas tank". Of course then, we went down a rabbit hole with our future.... increase in inverter, fuses, cables, etc.

I need to go with 2 12 v 100ah or one 200ah for size reasons….I don’t have an inverter now but was thinking the larger (200 BMS) would be better (headroom ?) for whatever I do…
Any thoughts anyone?