Where can I find a BMS that can manage 2 lipo packs?

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Oct 27, 2021
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I know this is more of an electricle vehicle/ebike forum, this is sorta off topic but batteries are still the same I guess.

I'm building a rover that needs 24v and at least 30A. Right now I'm using 2 of these battery packs in parallel for extra capacity.

Right now I'm taking the batteries outside for charging and keeping an eye on the voltages to make sure it doesn't over discharge while in use. However, I'm looking for a BMS that will allow me to just plug in a cable to charge the batteries and automatically make sure it doesn't go below the voltage cut off which is 3.2V for this battery pack.

I was told that this isn't a smart idea because the packs are in series first and then I'll put them in parallel, and I should do parallel first then series. I'm not sure if this is actually the case since I can still use the secondary cable to connect them in parallel, no?

If I cannot use the setup above, what's a better option?

I also found this battery pack which is overall less capacity but a single pack so no issues with parallel. I still need to find a BMS that fits my needs. Most of the ones I found are for li-ion cells where the cutoff is less than 3.0v.
 
karim-humanitas said:
If I cannot use the setup above, what's a better option?
Two BMSes.

BMSes do a lot of things, including protect against battery misuse (shorts, overcharge etc.) If it can be done to one battery but not the other, then the BMS no longer protects it.

Also if you have two series stacks connected in parallel, either you need 2x the voltage monitoring (which effectively means 2 BMSes) or you need to connect all the inter-series buses. And that IMO gives you more ways to screw up than you want. In other words, doing that and then adding a BMS will make the pack less safe, not more.
 
JackFlorey said:
Two BMSes.

Thanks for your reply. I forgot to mention that I have asked this question on reddit batteries before and they said not to use a separate BMS for each battery. It was posted by one person who said something about the flow leaking to the other cell if they don't charge/discharge at the same rate. Basically one would be charging the other.
I also read somewhere else the following " result in a huge inrush current when one of two BMS turns itself back on" but again, I don't understand why that would happen since nobody really posts their references when they reply.

If you check the attachments. battery.JPG shows how a single lipo pack looks. battery2.jpg shows what I thought I should be doing. I'm not really sure if this counts as parallel first or series first?
 

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karim-humanitas said:
I know this is more of an electricle vehicle/ebike forum, this is sorta off topic but batteries are still the same I guess.

I'm building a rover that needs 24v and at least 30A. Right now I'm using 2 of these battery packs in parallel for extra capacity.

Right now I'm taking the batteries outside for charging and keeping an eye on the voltages to make sure it doesn't over discharge while in use. However, I'm looking for a BMS that will allow me to just plug in a cable to charge the batteries and automatically make sure it doesn't go below the voltage cut off which is 3.2V for this battery pack.

I was told that this isn't a smart idea because the packs are in series first and then I'll put them in parallel, and I should do parallel first then series. I'm not sure if this is actually the case since I can still use the secondary cable to connect them in parallel, no?

If I cannot use the setup above, what's a better option?

I also found this battery pack which is overall less capacity but a single pack so no issues with parallel. I still need to find a BMS that fits my needs. Most of the ones I found are for li-ion cells where the cutoff is less than 3.0v.

A single LiPoly battery is refered to as a "brick" and I suppose two in parallel could be called a "pack".
What charger are you using? I am concerned with how you are charging, as you appear to have never learned about the care and handling of LiPoly.
First off, never discharge LiPoly to 3.2V as this is harmful to the battery and will greatly shorten it's service life, but more importantly, will create imbalances, that left unrectified, can create a dangerous condition. Most of us take great pains to not let any cells get below 3.6V, but the best idea is to adhear to the Depth of Discharge Rule which states "Only use 80% of cell capacity".
I would suggest you invest in a quality RC charger as any BMS's will not work well enough with what you trying to do. They are not designed for LiPoly architecture, not to mention that each cell can not be monitored accurately if the cells are paralleled.
A 120W X 2 charger would allow you to charge two bricks in a reasonable amount of time and a decent one is around $140. You would have to "break" the pack of course.
There are ways to "bulk charge" involving Mean Wells and Battery Medics which allow the pack to remain intact, and in the vehicle for that matter. But the hardware can be as expensive as a RC charger.
As for monitoring discharge values, since will be using a system of 24 Volts, your 24V controller should have a useful Low Volt Cut-off for that Voltage. Failing that an alarm could used.
Lastly, I would highly recommend you read up of LiPoly. plenty of info out there, either here or on the RC forums.
 
What your diagram shows is the right way to do it with a single BMS. That should work fine. Just make sure both packs are at the same voltage before connecting them to each other (within 0.1v or so).
 
motomech said:
What charger are you using? I am concerned with how you are charging, as you appear to have never learned about the care and handling of LiPoly.
First off, never discharge LiPoly to 3.2V as this is harmful to the battery and will greatly shorten it's service life, but more importantly, will create imbalances, that left unrectified, can create a dangerous condition. Most of us take great pains to not let any cells get below 3.6V, but the best idea is to adhear to the Depth of Discharge Rule which states "Only use 80% of cell capacity".

I know how to handle LiPoly. I'm using an ISDT D2. I don't want to discharge it to 3.2 particularly, somewhere between 3.3 and 3.5v. But most of the BMSes I find are made for LiFePO4 and li-ion cells which tend to have a lower cut off. I want to use up as much of the capacity as I can, hence the need to go as low as I can.

I currently have an alarm connected to the batteries when in use, and that's how I make sure it doesn't dip below the set voltage. So I have a good charger, I'm taking care of my batteries and everything. But this is not the whole story.

The whole story is this is for a rover project. I would like to build a battery changing mechanism to automatically replace the batteries for the rover and charge them automatically. So the way I was going to go about doing this is by putting a BMS on each pack (2 batteries). The rover will automatically pick up a fresh set of batteries, use it for a while. Once the voltage becomes almost at the cut-off, it will got to the station to get the batteries swapped. To make my life easier, I would like to only have 2 connections per pack instead of having to connect 18 different cables automatically.

With that being said, I think what I need to find is a BMS made for LiPos and hopefully one that has communication, so I can see the voltage through UART straight away instead of using another sensor there.

Not sure if that makes sense or answers your questions.
 
fechter said:
What your diagram shows is the right way to do it with a single BMS. That should work fine. Just make sure both packs are at the same voltage before connecting them to each other (within 0.1v or so).

perfect! thanks. Do you know where I can find a BMS for my situation?
 
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