Discharge balancing on Parallel batteries??

themyst

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Nov 30, 2018
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This question doesn't relate to an e-bike, but an electric scooter (Dualtron Spider)

Hi, I was reading through the sticky in regards to parallel battery setups, and my plan is to run two 16S5P packs (one pack is LG MJ1 with 10 cycles and is internal to the scooter, and one pack is Sanyo GA cells with no cycles) in parallel, both with their own BMS. Everything makes perfect sense to me (don't let the two packs resting voltages deviate too much from each other, etc)

The reason I decided to run the packs in parallel instead of as a second battery to switch between is due to the fact voltage sag is pretty rough on a single battery (it could draw upwards of 54A peak and 30A sustained, at maximum settings you can see 10V of sag) where running them in parallel would also run them cooler and have a lot more headroom.

My questions:

What exactly would happen if I charged through the scooter with both batteries connected in parallel on the discharge? If I connect 2x 2A chargers directly to the scooter which is directly connected to the MJ1 pack, would the MJ1 pack in essence, de facto charge the external pack? Given the total input current is no more than 4A, I can't see that doing any damage at all considering the scooter will be off during the charging process.

If this is confirmed to not be harmful, how would this question play if I were to up the input current to 8A? (The scooter's spec sheet indicates that up to 8.4A charging is acceptable). I don't necessarily mind charging the batteries separately while disconnected, but when one of your use cases would be to cover extremely long distances, sometimes doing so may not be plausible. Would not fully charge the cells if using the 8A charger, I have a Charge Doctor to cut the charge when voltage hits 64-64.5V.

Second question is if both batteries have separate input lines, can't we parallel those lines together as well so that we can charge from a single source with the discharge connector off, or is that a waste of time?

Thank you so much in advance for any guidance you all may provide!
 
What you are suggesting should mostly be fine depending on your wiring. I say mostly because I am not sure how you are going to keep you battery balanced at 64v. And, if you are going to give the GA cells 8 amps (not 4a) then you are pushing their standard charging limit.

The wiring is key to what you want. Post a drawing of your wiring so we can see what you are trying to do. It will help us give you a better answer.

:D :bolt:
 
e-beach said:
What you are suggesting should mostly be fine depending on your wiring. I say mostly because I am not sure how you are going to keep you battery balanced at 64v. And, if you are going to give the GA cells 8 amps (not 4a) then you are pushing their standard charging limit.

The wiring is key to what you want. Post a drawing of your wiring so we can see what you are trying to do. It will help us give you a better answer.

:D :bolt:

I have attached what I am thinking, all lines will be 12AWG, with EC5 connectors all around which should be plenty to support the current I am looking to push. The reason I chose EC5 over say an XT90 is because it is much easier to solder multiple lines into the cup on an EC5 connector compared to the half-cup solder points on an XT90.

And I think there might be a miscommunication regarding the 64V cutoff, the reason I would not full charge at 8A to 67.2 is because of the added heat and stress being added when full charging at 8A. At home, I would use 2x 2A chargers, one on each battery pack with the external disconnected from the parallel.

This leads me to my next point, to charge both batteries through one input (the one connected to the internal MJ1 pack), do I even need to parallel the input from the external pack, or will it charge both packs through the discharge line already connected if I were to plug it into the charging port connected to the scooter (internal battery)?

This won't happen often, I'm only asking because as I said in the OP, it can become real cumbersome charging on the fly with only one charger... when I am going for long distances, the time I have to charge won't be longer than an hour or so.

The sticky on this site says it should try and balance the resting voltage to be equal between both packs, but you do not want to have the voltage difference be too large due to the fact it will dump a crap ton of current into the battery with lower SOC.
 

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The simplest way to ask my question is, will I cause any damage if I charge through the scooter's charging port if I have both the internal and external paralleled?

Would the internal battery connected to the scooter's charging port also be sending current to the external battery in parallel to keep the voltages about the same?
 
Unless both BMS use only one set of charge/discharge leads you need to charge only with the charge side. Many BMS won't balance or shut off if charged thru the discharge side.

I don't see a concern charging both packs with 8amps if parallel. Concern is some BMS don't like much over 5 amps. While charging both packs the power should be split or at least close. When the charger reaches close to full charge they should taper the currant. Reason to charge till BMS shuts off the charger is so the cells are hopefully balanced. Cutting power early can cause cells to get unbalanced easier. Not saying they will if they are balanced from the start and of close capacity. 18650 cells once balanced well stay rhar way for a long time in most cases. I have cells from new HP packs that have stayed balanced for years now and no BMS. Maybe once a year I will balance them. Don't run almost all Lith cells and they stay balanced, at least mine have.

I have been charging my Multistars at 10amps for 3+ years with no problem. I don't have a BMS but do check them each charge. I now am running 2 sets of 16ah backs paralleled and they are doing great. Many miles and charges now and they are down about 1ah each. A short ride here now is at least 31 miles.
 
DAND214 said:
Unless both BMS use only one set of charge/discharge leads you need to charge only with the charge side. Many BMS won't balance or shut off if charged thru the discharge side.

I don't see a concern charging both packs with 8amps if parallel. Concern is some BMS don't like much over 5 amps. While charging both packs the power should be split or at least close. When the charger reaches close to full charge they should taper the currant. Reason to charge till BMS shuts off the charger is so the cells are hopefully balanced. Cutting power early can cause cells to get unbalanced easier. Not saying they will if they are balanced from the start and of close capacity. 18650 cells once balanced well stay rhar way for a long time in most cases. I have cells from new HP packs that have stayed balanced for years now and no BMS. Maybe once a year I will balance them. Don't run almost all Lith cells and they stay balanced, at least mine have.

I have been charging my Multistars at 10amps for 3+ years with no problem. I don't have a BMS but do check them each charge. I now am running 2 sets of 16ah backs paralleled and they are doing great. Many miles and charges now and they are down about 1ah each. A short ride here now is at least 31 miles.

Thanks for the reply! I do think the BMS should support reverse voltage, as regenerative braking will result in that to occur. Not sure what the amp limit is.

My plan wasn't to cut the charge off all the time, only when needed to charge quickly in a pinch when traveling long distances. When I'm back home, I'd be more than happy to charge it slowly to full, the correct way.
 
themyst said:
What exactly would happen if I charged through the scooter with both batteries connected in parallel on the discharge?
It depends. Most times none of the below is harmful...but if cells are badly unbalanced or there is a sufficient defect in a cell, etc., it could be.

If both packs have separate charge and discharge ports, then the BMS cannot prevent overcharging thru the discharge port (it can only prevent it thru the charge port).

So...if the external pack reached full on a cell, and needed to shut off discharge, it could not, and hte cell would continue overcharging, until either something catastrophic happened, or the internal pack shut off *it's* charge port which would then disconnect charge current from the whole system. As long as the internal pack's voltage is not higher than the external pack's, the EP would begin to bleed the high cell(s) down at that point...but current would also still flow out of the IP into the EP, albeit at a low rate, probably low enough to not worry about as long as the EP was already nearly full when it tried to shutdown, but if it was not, then it would probably swamp the balancing process.

If the EP is about full, and the IP is not, but tries to shut off charging because it needs to balance, it is going to still receive charge current thru *it's* discharge port out of the EP's discharge port, as it slowly drains down the high cells inside itself, because it's making it's voltage lower than the EP's.


If the EP has the same port for charge and discharge, then only the last is a potential problem.

If both packs have the same port for charge and discharge, then there is no problem simply paralleling them.


If I connect 2x 2A chargers directly to the scooter which is directly connected to the MJ1 pack, would the MJ1 pack in essence, de facto charge the external pack?
Yes, thru the external packs' *discharge* connector (assumign it is a different port than the charge port). See above for the problems with that.





If this is confirmed to not be harmful, how would this question play if I were to up the input current to 8A?

That depends on the batteries. If they are actually capable of a charge rate that high, then no problem. If they are not, then they will heat up more during charge, and age faster.

Additionally, if they are parallelled for charging (like you ask about below), then if they had been sharing about equal current at max input rate, and one cuts off charging for whatever reason, the other would suddenly be able to draw more current (up to whatever it's internal resistance/SoC allows at that point). Whether that's a problem or not depends on the packs' max rate--if you depended on the split current to keep them safe...it wouldn't necessarily be able to do so.


Second question is if both batteries have separate input lines, can't we parallel those lines together as well so that we can charge from a single source with the discharge connector off, or is that a waste of time?
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You can certainly do that, and it's perfectly safe to charge that way. Each BMS will disconnect itself from the charger as needed during charge.

The only potential problem is similar to the other one above: this is not a unidirectional port, so if a pack shuts down it's discharge port during a ride, to protect itself, it will still be able to discharge out of it's charging port (even if it shuts off the port! because of the parasitic diode in a FET), into the charge port of the other pack, thru that packs' BMS and it's discharge port, and into the scooter.

So the pack trying to protect itself can't. If it's a low cell, it'll keep getting lower. If it's some other problem, it'll continue. Additionally, if it has also shutdown the charge port for whatever reason, the charge port FETs will begin heating because they're not "on", and it's just a diode to the current flow thru it. If the current is low it's not a big deal, but if it goes up too high it can cause the FET to overheat and fail (or even unsolder itself from the PCB).






So...to be safe, you'd want to separate the charge ports when discharging, and the discharge ports when charging. But they can be paralleled just fine whenever performing the pack operation the ports are designed for.
 
amberwolf said:
So...to be safe, you'd want to separate the charge ports when discharging, and the discharge ports when charging. But they can be paralleled just fine whenever performing the pack operation the ports are designed for.


Thanks for such a detailed response! Much appreciated. The only time I would be engaging in charging the external through the discharge port would be when I am trying to top off during a LONG ride. Given that I will be more or less using the 8A charger exclusively for this situation, I do not think the cells would go dangerously out of balance given the nature of top-offs while on a long trip, and I have a device which will terminate the charge before the 67.2V cut. I currently have it set to 64.8V.

I would charge them normally through their respective inputs with the parallel disconnected when I get home.

The capacity spec of both packs should be more or less the same; the MJ1's are specced at 3500mah per cell (but tests show they are closer to 3400-3450) and the GAs are specced for 3450 mah with tests showing that to be mostly accurate, so any transfer of energy between one pack to another would be marginal and shouldn't be cause for concern I think.

I know the internal pack has 10 charge cycles on it, which I do not think should affect capacity all that much, if at all, which SHOULD, in theory, mean both packs would have similar resting voltages when they need to be topped off. I can always bring a multimeter to confirm the resting voltages after a long run.

Given the total capacity of both packs is around 35Ah, the 1C with both packs combined is 35A, and 8A charging would result in .22C, which is a pretty safe rate of charging.

Does this sound like a plausible use case? Thanks!
 
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