Charging two batteries off a single charger OK?

Hi,
I have built two identical 13S5P lithium ion battery packs (same cells, same BMS) to power mine and the wife's ebikes. I have also built a CC/CV charger, capable of delivering up to 10A at 60V, which I aim to generally run at 2A (per pack) at 53.3V (max cell voltage 4.1V) to maximise cell longevity, and occasionally run at 54.6V (max cell voltage 4.2V) to perform periodic BMS cell balancing. Ideally, I'd like to charge both battery packs from the same charger simultaneously, wired in parallel at the charger output. But would like to know if there are any issues with this approach, given each battery pack would likely have a different starting voltage when charging is initiated. Or is there a need to have a dedicated charger for each pack to avoid unforeseen issues.

Many thanks.

Someone with more experience will be along soon.. but....

yea should be fine. Charge rate will be halved if the resistance of the packs at the nodes is shared.... Each pack will only take the current it needs.


There are certain hobby chargers that can accommodate charging of many packs at once. I mean, I can charge 12 (6s) 5000mAh packs at 2C with my hobby charger. In parallel.

Charging two BMS managed packs of the same voltage should be fine, really, regardless of the capacity of the packs. The charger will charge whatever is hooked up downstream, it doesnt care whether or not it is one pack or two... and the packs wont care as long as they can take the full current of the charger alone.. they will just share the load until full. The BMS(s) (onthepacks) dont care unless there is an overcurrent on charge event.

For example, Every Diesel truck with two batteries shares one (generator) charging source. Never any sort of problems there, charging 12 cells in two groups of 6 in parallel.

Yes, lead acid example, but it is still how the cells would work, would work the same, if lithium.


Yes the danger(s) is : hooking up two packs at different SOC. The surge of energy from one to the other may trip the BMS. Or parallel charging a number of cells in parallel with no protection measure in between ( fuses or BMS). The possibility of a fault is unprotected in that case.

No
As after riding your packs will be at different states of charge. The voltage from the higher voltage pack will flow to the lower voltage pack at high rate.
Best to use two chargers on one power strip. It just safer so everybody can charge battery safely.

Possibility is to place directional diodes to each charge connection.
You want to directionally isolate batteries from each other.
Full charge amperage would flow to lowest voltage battery till nearing equal voltage, then should gradually transition to charging both batteries with equal voltage and amperage, relevant to capacity (if similar IR).

DrkAngel said:

Possibility is to place directional diodes to each charge connection.
You want to directionally isolate batteries from each other.

Click to expand...

This is what I would do. Then it doesn't matter if you have a different SOC in each pack.

Great feedback guys. Thanks.
So, in summary, it should be fine to charge both at the time from the same charger provided I have a suitable diode across each battery pack charge connection to prevent the risk of any “back charging” from higher pack voltage to the lower voltage pack. So just need to add a couple of diodes to my charge cable that can handle >2A at 60V.

Just as a caveat, you should confirm that your BMS will still activate balancing mode with the slightly reduced charge voltage (due to voltage drop across diode). And that the power dissipation across the diode won't cause heat problems. You could use a Schottky diode to minimise those effects.

Hot Toddy said:

So, in summary, it should be fine to charge both at the time from the same charger provided I have a suitable diode across each battery pack charge connection to prevent the risk of any “back charging” from higher pack voltage to the lower voltage pack.

Click to expand...

Not "across" the pack, which implies placing one lead of the diode on the pack positive and the other on the pack negative.

It needs to go between one lead of the pack and the same lead of the charger, in the polarity that prevents current flow out of the pack.

So just need to add a couple of diodes to my charge cable that can handle >2A at 60V.

Click to expand...

The diode must be able to handle *at minimum* the highest possible voltage difference between the most empty either pack will ever be, and the most full either pack will ever be. So 60v is not really needed, but won't hurt.

The diode must be able to handle *at minimum* the highest possible current that will flow between the charger and that particular pack. If it is a 10A charger, then the diode has to be able to handle 10A, minimum. (unless you choose to lower the current limit of the charger, or the charger doesnt' actually ever put out 10A even when only one pack is connected / charging and is at it's very lowest voltage).

The diode must *also* be able to handle the worst-case power dissipation load (watts), which is the highest voltage drop the diode will create between the battery and the charger, multiplied by the highest possible current that can ever flow (which will happen at that voltage difference).

Let's say the diode drops 1V at the highest current, and that highest possible charge current (assuming you don't change anything) is 10A, so 1v * 10A = 10W. So you'd need a diode that can handle, at minimum, 10 watts of power dissipation. If the voltage drop is less, then you need less power dissipation. If the voltage drop is more, then you need greater.

If you get diodes in the TO220 or similar package, they're easy to mount to heatsinks, which will improve their ability to dissipate the heat, if necessary.

Some options are in this list (I restricted via various options down to 6, but you can change the filters used)
https://www.mouser.com/Semiconductors/Discrete-Semiconductors/Diodes-Rectifiers/Schottky-Diodes-Rectifiers/_/N-ax1mj?P=1yuoc70Z1yuoc6nZ1z0y481Z1z0z63xZ1z0y4ci
whcih is just the single-diode-in-a-package version, which you can use on either the positive or the negative wires.

Then this is the dual-diode-per-package version, so if you have lower power requirements you can use just one device with two diodes in it to do both batteries; they are dual-common-cathode so you have to use this with teh common pin on the charger negative, and the battery negatives go separately to the outside legs.
https://www.mouser.com/Semiconductors/Discrete-Semiconductors/Diodes-Rectifiers/Schottky-Diodes-Rectifiers/_/N-ax1mj?P=1z0y481Z1z0z63xZ1yuoc70Z1yuoc6nZ1z0y4bt



If you're using lower *total* charger current limit (say, 4A with the intent that this be "split" between two packs), then you can use diodes that only need 4A and 4w capability (assuming 1v drop).


A final consideration is your charger total voltage: With the diode in place, it may require some adjustment. The voltage drop across a diode drops as the current drops, but it doesn't reach zero, so you may need to experimentally adjust teh charger voltage to end up with the desired actual full battery voltage.

Thanks for the clarifications. I will be current limiting to 4A (2A per pack). The max voltage differential between the packs is unlikely to exceed 20V. So will go with Schottky diodes that can handle 40W and increase the charger voltage to account for the (~0.3V) drop across the diode.

Thanks again guys.

Hot Toddy said:

Thanks for the clarifications. I will be current limiting to 4A (2A per pack). The max voltage differential between the packs is unlikely to exceed 20V. So will go with Schottky diodes that can handle 40W and increase the charger voltage to account for the (~0.3V) drop across the diode.

Thanks again guys.

Click to expand...

Lowest voltage pack will, initially, draw the full amperage. So, best to figure each diode capabilities for the full 4A.
(Could be moderated by light gauge secondary, after split, charging leads. But, recommended Lithium charging amperage is typically .5C = 5A for 10Ah battery, I haven't seen evidence that further lowering charge rate has any substantial benefit. ... ?)

Hot Toddy said:

Thanks for the clarifications. I will be current limiting to 4A (2A per pack). The max voltage differential between the packs is unlikely to exceed 20V. So will go with Schottky diodes that can handle 40W and increase the charger voltage to account for the (~0.3V) drop across the diode.

Click to expand...

The diode has to be able to handle the full 4A in that case, because there are various instances in which one pack or the other will not draw any current (and you might even know it), so all the current would flow thru the diode of the other pack.

For the specific voltage drop, you should test for it in your specific setup at the worst-case highest current that will happen. The data sheet for the diode will get you pretty close, but there are always variations. Probalby not enough to worry about...but better to test and be certain than assume and end up with a potential problem later.