Two identical batteries in parallel on the same bike

Someone asked a question about charging 2 batteries with one charger,. It prompted me to ask a similar question, but with other specifics. I can build bikes, but I'm a bit electro-dumb.

I have the EM3ev 14s7p hardcase rectangle pack. It has ~70 cycles on it. Just bought a second one, exactly identical. I want double the range (2x the fun!) I was getting before.

What I want to do:

Strap down and connect both in parallel (using XT90 parallel harness) to the controller of my X1 pro, cyclone 3000, or bbshd.

Any issues here? With Discharging?

How about disconnecting both, and charging individually? I plan on getting an adjustable 2-12a charger, that feeds through the XT90 on the battery, instead of the low amp Anderson. The Smart BMS allows this.

If I keep both connected the whole time I use them, any issues with charging, then? I know it's not ideal having 70 cycles on one, and 0 on the other. I need to pass this by you guys as a sanity check, before blowing anything up.

Thanks in advance!

2 identical batteries in parallel...
It is just as simple as your think it is.
They must be at the same state of charge when you parallel them, then you can charge both together if you wish to. Charging them separately would be faster, unless you buy a charger twice the Amps of those that you have.

Better unplug battery from controller when not in use, Lico especially.

70 cycles is not a problem, yet I would ride with the new one alone a few days to cycle it before parallel assembly.

Like M.R. said...state of charge is important. That means they should be at exactly the same voltage when you connect them to avoid an in-rush of current to the undercharged battery. Charging them connected together in parallel with one charger is a good way to make sure they are the same voltage when you attach them to your controller.

There are some caveats.

--During charging, if they have separate charge and discharge ports, then if you don't disconnect the discharge ports, then when one BMS tries to shut off charging current to it's pack because of a full cell, it can't--it's still getting charged thru the discharge port from the other pack and charger. If there is a problem with the pack this can cause overcharging. (the consequences of which depend on the seriousness of the problem with the pack)

It's not likley to be much of an issue unless there's a serious imbalance, or one pack is significantly different in capacity than the other.

--During discharging, if they have separate charge and discharge ports, then if you don't disconnect the charge ports, then when one BMS tries to shut off charging current to it's pack because of an empty cell, it can't--it's still getting discharged thru the charge port from the other pack. If there is a problem with the pack this can cause overdischarging (the consequences of which depend on the seriousness of the problem with the pack).

The latter problem isn't likely to be a problem if you setup the system normally, with the controller LVC higher than the battery BMS LVC. (since the BMS LVC is an "emergency cutoff" as a last ditch effort to prevent cell damage, while the controller LVC is the normal system cutoff. It's only a potentially serious issue if the BMS LVC is the only cutoff, *and* there is some fairly serious imbalance that would cause one pack to cutoff significantly before the other one.


If the packs have the same port for charge and discharge, then neither of these are likely to be an issue.




For packs with separate C/D ports:

If you don't want to disconnect anything (via switches or connectors), you can use diodes (which have to support the full discharge current of the system, and have to have a reverse voltage higher than the pack's full voltage) on the discharge ports as noted above, as long as you don't use regen on the system. The line on the diode package goes on the controller side of the positive wire, or the battery side of the negative wire, so the arrow points in the direction of current flow.

If you have to use regen, you can't use diodes. You're then stuck with disconnecting the discharge ports from each other in some fashion.

The easiest way for that is to just put a plug on one pack that's easy to connect/disconnect. But you want to make sure it *has* to be disconnected in order to charge the pack.

So you can use a charging port connector that is actually enough pins to also run a jumper wire from just one wire of the discharge port of one pack to the controller (two pins). So, four pins total if you're using one charger for both packs, or six pins if independent chargers.

Then you unplug the jumper connector used for discharge, and connect the charge connector for charge (which does not have the jumper wire in it).




For more perspectives and info, the topic in general has been discussed in some of the many such threads, most of which are in this list
https://endless-sphere.com/forums/search.php?keywords=parallel*+batt*&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
though not everything there is relevant.

Regarding charging thru the discharge port...be very certain that the BMS actually can shut off charge on that port.

If it cannot, it has no way to stop the charger from overcharging cells in an unbalanced battery situation, and you would never want to charge thru that port whether single or paralleled.


If it *can*, then this effectively makes these packs a single-port charger, and you would wire only the XT90 connectors in parallel, and leave the others capped off so you don't hook them up to anything (or each other), and then only charge and discharge thru that paralleled XT90 port.

Awesome, thanks guys. It sounds like there won't be any issues if I keep them in parallel for all charge/discharge cycles. The BMS has a 12a charge current limit through the XT90 discharge port. And I don't see myself using one of the packs by itself after the upgrade. This should be interesting on the BBSHD high racer recumbent: 120 mile range at 30 mph. My lower back hurts just thinking about it

Just be sure to get enough weight low and in the rear of the bike to compensate for the weight of the motor high and over the front of the bike.

Good batteries offer closely matched cells. 2 good batteries can be made from cells that don't match one battery vs. the other, and it's possible that there may be enough difference where there are issues when balance charging.

Point being, I'm on board with running them in parallel, and even charging to 90+%. When balance charging though, I would charge them separately.

If you separate for charging, and have two chargers, you'll be done in half the time.



Or a quarter of the time, if that's your thing

Done in half the time because you are charging at double rate because of the split pack. One of the main reasons I use a split pack. Another reason is the two chargers are cheaper then one high amp charger. Downside is if one half the pack gets lower then the other, can lead to an under charged pack once the LVC of the controller is hit. Thats if you dont run a BMS. I have run into that circumstance before.

Voltron said:

If you separate for charging, and have two chargers, you'll be done in half the time.

KIMG0085-576x1024 (1).JPG

Or a quarter of the time, if that's your thing

Click to expand...

Yes...I checked often to make sure the 36v modules were matched before the series hookup. And I spent time getting the chargers very matched on their cutoff points.