Charging two batteries with one charger

If i have 2 batteries and 1 charger can i charge them both at the same time with a double connector like this?


Currently when i charge my batteries i charge one battery and then after some time i have to move the connector to the second battery to charge that.

I dare not to try and use such a double connector until i know if it is save or not and why.

The batteries are like this.
They both contain a BMS inside the box.

I think there are some sound reasons that you should not charge both batteries in parallel, but it probably depends on the battery chemistry and how the battery and charger work together. But the real point of my response here is what are you trying to gain? Charging both batteries at the same time would take twice as long as charging a single battery, hence you can charge both batteries just as quickly, one at a time, as charging both batteries at the same time.

Also, if you charge both batteries together, they need to be at the same voltage before you wire them together in parallel, which is what you are doing. If you are using the batteries in parallel, then charging them in parallel might make sense, but if you are using them one at a time and/or on different bikes (or whatever) their voltage will not be the same when you are ready to charge them.

In the old SLA days I used multiple batteries wired together first in series (four for 48 volts) and then in parallel to increase amp hours. Of course it required a two wheel trailer behind my trike to haul 100 pounds of batteries. With the various Li ion batteries in use today with their BMS systems I wouldn't consider doing something like that.

Well God knows what that abortion of a connector is doing there

IEC C13 appliance connector like a PC power supply cord, but on the **DC output** side?????

**Big red flag!**, highly illegal in all civilized jurisdictions and for good reason, dangerous AF

please link to where you got that pic. . .


But, setting that aside, in general no problem, long as the two batts are parallel connected, same model, rated capacity and similar SoH.

If not, then charge separately.

LI chemistries other than LFP and LTO, be extra cautious, monitor V&A, not unattended, high fire risk.

No to many ways to go wrong. Some very bad. Just need two chargers.

It may be possible under certain controlled conditions (I do it), but as others have pointed out there are a lot of ways it can go bad. If the two batteries are in series, it will immediately cause a short and blow something. If the two batteries are separate, they need to be at the same voltage when connecting or you get a surge of current that could blow something or cause a fire.

Best bet is to get another charger.

Yes, charge in parallel like that only if you are running naked cells, with no bms at all. I do this sometimes with my RC packs, but only if I know they are not out of balance too much, and are at the same voltage when I connect.

FWIW, its not faster, the charger puts out what it does. Takes the same time to fill both. Getting a second charger when you can afford it will get you charging twice as fast, and give you a back up if one charger goes bad.

I added pictures of the batteries in the first post.
Each battery contain a BMS.

One reason why i would like to charge both batteries at the same time is so i dont have to remember to move the cable.
Even if i did want to move the cable to the other battery manually i would not be able to know exactly when the charging of the first battery have ended.
Suppose you come home late at night and set one battery to charge and go to sleep the next day you wake up to only one charged battery instead of two.

Also you might not be the same place as where the battery is being charged in case the scooter is standing outside where there is a socket.

Since each battery contain a BMS i would think that it does not matter if the batteries are at the same voltage. Right?

More then one of you say the charging wont be faster but i think it might be faster (then charging one battery at a time with only one charger) because when the battery is charged to about 80% the current goes down so lets say you have two battery, one is 50% charge and the other is 80% charged.
Then i would think the one battery is being charged maybe almost as fast as if it was the only battery connected because the other battery is only being charged with little current as it is close to 100% charged.
Plus if you only charge one battery at a time and have to move the cable manually you have to be there at the exact time the battery is fully charge else you get a period of no charge.

Yes its a bit silly/dangerous that the connector for charging the battery with 48VDC is almost identical to the 220VAC going into the charger.
Note that earth is turned differently.
But i think i have even seen scooters using 48VDC connectors with the exact some connection as 200VAC.
My i have a charging cable that accepts both earths.
The website dont have that specific charger anymore.

scootergrisen said:

Since each battery contain a BMS i would think that it does not matter if the batteries are at the same voltage. Right?

Click to expand...

A BMS is good for saving you from unintentional stupidity. It'll try to keep you from draining the cells too far, from drastically overcharging them, etc. A BMS is generally not robust enough to save you from active, intentional stupidity.

Paralleling lithium batteries at different voltages is how you kill your pack(s) and quite possibly start a fire that you can't easily extinguish. Yes, even with BMSes. Don't do it.

Me so sad if i cant charge two batteries with one charger.
I have two chargers but i want to have one of them permanently mounted inside the scooter where it cant be removed easily.
And then one at home.
So i dont have to move them.

If anyone knows any scooters that can charge two batteries at the same time i would like to know so i can see how they do it.

It seems you haven't given us sufficient information to help you, but the answer may already be present.

If the two batteries are not wired together, then you must be using one at a time and switching to the other when the first one runs down. If this is the case you have two choices, namely first charge one and then charge the other, or install both chargers in the scooter and when you have access to an outlet run both chargers and charge both batteries.

Many commuters on the forum have found out that chargers don't survive for very long when carried on a bike all the time due to vibration and bumps. Perhaps on a nicely suspended scooter that won't be a problem, but it could be. If you always commute to the same place you could keep two chargers there and two chargers at home, but of course that means you have to buy two more chargers.

I'm still interested in hearing from people who know about using only 1 charger to charge 2 lithium-ion batteries in parallel.

I have been looking for info and things like electrical wiring diagrams and pictures from scooters that comes with 2 batteries but not easy to find anything other than a picture of an open seat where you can see the two batteries.

Here are lots of pictures of the things involved. Charger, battery, BMS, cells etc.
https://scootergrisen.org/forum/viewtopic.php?f=19&t=3601
https://scootergrisen.org/forum/viewtopic.php?f=19&t=3614
https://scootergrisen.org/forum/viewtopic.php?f=19&t=3612
https://scootergrisen.org/forum/viewtopic.php?f=19&t=3854
https://scootergrisen.org/forum/viewtopic.php?f=19&t=4170
https://scootergrisen.org/forum/viewtopic.php?f=19&t=3606

they should be at the same voltage before you connect them together.

if they aren't, then the higher voltage one will immediately pour current at the highest possible rate for the difference in voltage and the total resistance between the cells in one and the cells in the other, into the lower voltage one as soon as it's connected, possibly destroying the bms of one or both by overheating the fets on the charge port, if that current is higher than they are rated for.

since fets often fail shorted, not open, then the bms can "silently fail", so that it now can no longer stop charging from happening even if the cells are overcharged and telling the bms to stop. it's silent because you have no idea this has happened, until something goes wrong inside a cell because of it and there is a fire.

whether any of this happens or not depends on many things, which you dont' have control over, other than ensuring they are always the same voltage before connecting them together, and never connecting them together if they are different voltages.



also, if they have separate charge and discharge ports on the bms, you need to disconnect the discharge ports from each other during charge, if they were connected before, or the bms cannot prevent overcharging because current will still feed in from the other pack that's still charging thru the discharge connectors of each pack.


if they have the same port on the bms for charge and discharge, it doesn't have that problem.



there are numerous other threads about parallel charging (and discharging) batteries that you can look up and read that have similar info, and probably more detail, and probably more info about other issues i didn't think of in the couple minutes i typed this up.

The BMS's have separate terminals for charge and discharge.

It seems to work in such a way that you can't charge and discharge at the same time.
If nothing is connected to the battery and you try to measure voltage between + and - there is no reading.
There is a thin wire that needs to be connected to + before you get the reading. Before discharge can happen.

How can one battery discharge into the other battery if...
1: nothing is connected to the discharge (PACK-) terminal.
2: discharge is being prevented because the thin wire is not connected to +.

So if what you are saying is correct then the terminal used for charging (CHIN-) can also be used for discharging?

How about having diodes so the current can only flow from charger to batteries and not between batteries?

From what i can understand the problem is not that the charger is connected to 2 batteries.
But that in doing so with a Y-cable it also courses the two batteries to be connected with each other.

So could the batteries not be isolated from each other some how?
With diodes or something.

Or perhaps have both batteries connected but only charge one battery at a time.
So when battery 1 is fully charged it would switch to battery 2.
Or have a timer that switches battery each 30 minutes or so they get charged about the same rate.

You could isolate them with diodes but there would be some voltage drop in the diodes that would need to be compensated for. The lowest battery will tend to take all the charge current until both packs are equal, then it should be even until they reach full charge. Diodes are pretty cheap.

If both batteries are always connected during charge and discharge and start out equal, the charger will stay happy.

That Y connector is meant for discharge only and not for charging and can only be used when batteries are of equal voltage. Yes need to be same voltage.
What no batteries are those ?

999zip999 said:

What no batteries are those ?

Click to expand...

What do you mean?

If i have something like 2 flash lights or mobile phones that can be changed with USB i would be able to charge them in the same USB connect with a Y cable like this right?
Not that i have done it but i would assume that i can.

So if i can do that why would i not be able to do the same with a charger and batteries for a scooter?

Sorry at the beach the sun's so bright I can't see my screen. What batteries are those ? I know this thread is old but it would make a difference what batteries are inside those boxes. I think that cable is for discharging the one on top.

What do you mean discharge the one on top?
The black box is the charger.

The batteries are lithium-ion.

The charger with the Y-cable was sold for a cargo scooter VGA e-Mover with these big batteries.
Perhaps the scooters was only sold to postal and delivery companies etc. because of the high prices.
So i have never seen anyone with such a scooter/charger and have not seen the Y-cable in use.

It might be worth it to just buy a second charger and not worry about connecting them. A lot of bad things can happen if the connection isn't done right.

My question is not so much to figure out a solution for me to buy a second charger but if it can be done with one charger.
And if it can't why not.

It seems some write that current will go from the battery with the highest voltage to the battery with the lowest voltage when the y-cable is connected.

But the BMS's in these batteries have separate terminals for charge and discharge and you even have to open for the discharge with a thin wire. So I don't think that current will flow between the batteries if the Y cable is connected.

The charger follows a precise charge protocol that is based on the battery voltage.

Unless both batteries are exactly at the same voltage and will behave the same throughout the whole charging process, there is no way for the charger to do the correct thing for both batteries at the same time.

So as was said before, may work in some situations, but could also go very wrong in others, so probably not worth the risk.

When the bms:es are on the batteries will charge/discharge each other if they aren't at the same voltage. If the bms:es are off they wont..
Are the batteries discharged in parallel? In that case they should be at the same voltage and I dont see a reason not to charge them in parallel.

aslong as both packs are identicle voltage/stae of charge id say this was ok.
but generally no...not good.

you could get a second charger instead, half your charging time.

and youll still have a charger you can use if one breaks.