Replace a BMS in 52v 20 ah battery pack

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Raider   100 mW

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Replace a BMS in 52v 20 ah battery pack

Post by Raider » May 03 2021 9:44am

Hello again.
This time I have a different question all together.
It's about battery packs and the posibility to change the BMS from what was delivered with the pack, to a different one.
I had a small accident when soldering a BMS. While desoldering a couple of bigger cables my soldering iron made the solder form beads as soom as it melted and two of those beads rolled down the PCB and onto the table. At least i thought they did. And to begin with I saw only one bead. And yes, my soldering iron sucks. Even if it is brand new. But that's what we have to finght with here in this forsaken country.
So I checked the PCB for any solder beads on the run before I closed it and couldn't find any.
Connected the power, switched on and the smoke came out..
Turned out that a bead has lodged it self under some cooling ribs that was impossible to see without removng the ribs them selves.
So, I ordered a new BMS.
Of course, it takes a few weeks for it to arrive, but this time the screwed up norwegian mail had done it again. They had actually returned the BMS to sender from the area postoffice without it ever coming close to me being able to pick it up at the local post office.
Theese guys p....es me off somethng royally. Norways best at..... Incompetance... Jeez.
So, I had to reorder and I now have to wait for at least three more weeks.

So, I was thinking. Isn't it a easier and better sollution to this?
I know many people are making their own battery packs, so I wonder. Could I buy another BMS, fitting a 52v 20ah battery, and replace the one originally in the pack?
If so, what kind of BMS would you recomend?
And also, since I use two batteries in parallel, should it be the same BMS in both batteries to make sure they stay as equal as possible when it comes to charging and power use?

docw009   100 kW

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Re: Replace a BMS in 52v 20 ah battery pack

Post by docw009 » May 03 2021 11:26am

Raider wrote:
May 03 2021 9:44am
So, I was thinking. Isn't it a easier and better sollution to this?
I know many people are making their own battery packs, so I wonder. Could I buy another BMS, fitting a 52v 20ah battery, and replace the one originally in the pack?
If so, what kind of BMS would you recomend?
And also, since I use two batteries in parallel, should it be the same BMS in both batteries to make sure they stay as equal as possible when it comes to charging and power use?
Isn't this what you're already doing? You have a BMS on order and you will install it when it arrives, but you don't want to wait a month for shipping. So you want install a new BMS from someone else, but won't you have to order that too?

Personally, I like the Daly BMS which come in a red metal case, with the heavy leads already soldered on. Those heavy leads still need to be connected to the battery, but I use butt connectors , which can be crimped or soldered,

Raider   100 mW

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Re: Replace a BMS in 52v 20 ah battery pack

Post by Raider » May 04 2021 3:31am

docw009 wrote:
May 03 2021 11:26am
Isn't this what you're already doing? You have a BMS on order and you will install it when it arrives, but you don't want to wait a month for shipping. So you want install a new BMS from someone else, but won't you have to order that too?
No, it's not really what I'm doing. Simply because it is a big difference between waiting for a shipment for 3 weeks o a month, and only waiting for a few days if ordered from somewhere in europe.
docw009 wrote:
May 03 2021 11:26am
Personally, I like the Daly BMS which come in a red metal case, with the heavy leads already soldered on. Those heavy leads still need to be connected to the battery, but I use butt connectors , which can be crimped or soldered,
Ok. Thanks for the tip.
I would need to use the same type of BMS on both batteries, right? To maintain balance?

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dogman dan   100 GW

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Re: Replace a BMS in 52v 20 ah battery pack

Post by dogman dan » May 04 2021 7:33am

This gets complicated and beyond my limited EE knowledge.

You can parallel two batteries with same voltage, but different bms', at the discharge wires. But you should un plug, and charge them separately. Then parallel them again to discharge.

Or, you can permanently parallel both identical voltage and chemistry batteries. Each tiny bms wire would be replaced by a larger wire to parallel each cell group of battery A to battery B. This is to allow flow of larger current back and forth, as you discharge one pack may take more load than the other so the wires need to be bigger than a thread.

Also both big discharge wires would be paralleled permanently. The bms sense ( thin wires) wires would attached to the fat wires paralleling each cell group. It won't matter where they connect too much, but ideally in the center of the connecting wire. Same length wire to each cell group is what I mean.

Then the fat wires connect to the bms to complete, again, ideally each pack has the same length wire to the bms location, so voltage drop across all wires to the bms are equal for both packs.

I found it easier to just parallel to discharge, and then charging separately meant I could use two chargers for a faster refill.

docw009   100 kW

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Re: Replace a BMS in 52v 20 ah battery pack

Post by docw009 » May 08 2021 9:26pm

I agree with dogman. Recently, I've been running a motley assortment of small 36V packs in parallel. All kinds of packs. Some are 10x2. Some are 10x3. Some from the same seller. I put the discharge outputs in parallel. I take them apart to charge separately, unless it's a single port battery where charge/discharge are combine. Then I leave them together.

If the packs are different capacities, the larger pack supplies proportionally more current. I know this because I've put two watt-hour meters on both packs before the tee-junction. Some folks have written, that if the packs are different capacity, some of the current from the larger pack goes to charge the smaller pack, and my watthour meters won't detect that because they don't measure reverse current. I'll have to find my third meter ( I have three) and put it after the tee to check on old Kirchoff. Professor Kirchoff developed the rules for current flow in the 19th century.

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E-HP   10 MW

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Re: Replace a BMS in 52v 20 ah battery pack

Post by E-HP » May 09 2021 10:31am

OK, after reading through the lamenting about your soldering skills, complaining about the country you live in, and the sentence or two related to your batteries, I think I've figured out what you may be asking for help on, in order, based on your post:
1. Ya, your soldering sucks. You should practice and be more careful. No other advice.
2. Ya, your country sucks. You could move, but who's going to move to another country over an ebike? Just face the fact that your country sucks, or deal with it. Maybe think about something nice about your country, and focus on that instead.
3. Can you buy another BMS to replace the fried one? Yes; easiest to get one with the same specs.
4. Do the two packs need to have the same BMS? No, they will function independently of one another and protect their respective packs. However, if one trips due to LVC or current, that may cascade and cause the other battery's BMS to trip when it has to supply all of the current. If you bulk charge them, through the paralleled discharge conductors, then the BMSs will likely not be part of the equation, but you haven't described how you intend to charge them.

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