Hey everyone, new member, a brief question: recharging individual batteries in series

[Waylander]

Hey guys, I've been a member of an ebike forums for a while but I could do with some core electronics info. Its been a while since my electronics GCSE course in school, but I'm hoping to wing my way through the ebike project I've been working on. Basically I'm changing the battery array in my ebike from 5 x 12v SLA batteries to 5 new li-ion batteries instead.

The new battery array comes out at 60v30ah as the 5 x 12v batteries are running in series. But instead of running a single 60v charger I have ordered 5 x 12v chargers instead. Can you guys check out my basic sketch at my website here **broken link removed** and advise if the individual chargers will bridge with each other and fry them or will they simply connect up with the appropriate negative terminals?

I've googled it but had no luck so perhaps if I can get an answer here, others will benefit from it too. Thanks for the advice in hand.

Cheers
Paul

 

[JimB]

Welcome to the forum.

But instead of running a single 60v charger I have ordered 5 x 12v chargers instead.

Why did you do that? Sounds like the hard (and expensive) way to me.

if the individual chargers will bridge with each other and fry them or will they simply connect up with the appropriate negative terminals?

Your terminology is a bit off, but lets try and be positive (unintended pun there).

I assume that your sketch shows five mains powered chargers, one connected across each of the five batteries.
This will work IF and only IF the charger outputs are completely floating.
By that I mean that there is no connection between the output leads of the charger and mains earth, and no connection between the output leads and the metal case of the charger (if the chargers do have metal cases).

JimB

 

[audioguru]

Multi-cell Lithium batteries used for model airplanes, helicopters, cars and boats are charged with a "balanced charger" that makes sure that each cell is not dangerously over-charged.

 

[Waylander]

Hey Jim sorry for the somewhat vague description and thanks for the prompt input. I purchased 5 chargers because I can charge the 5 x 12v batteries simultaneously in 2-3 hours rather than waiting 4-6 hours if I were to have taken a single 60v battery. There were a few reasons I took 5 x 12v with 5 x 12v chargers, 1 was that the chargers were pretty cheap from china, like £13 a piece and 2 I house a battery under the seat of the ebike, as the 5th battery of the bike wont fit in with the other 4 in the foot well's battery bay.

Does floating apply here with me using 5 chargers instead of one? and if I were to break the 60v series wiring to the controller with a switch could I then have one set of wiring from the battery heading off in series and another heading off to its own 12v charger that will be hooked up on the outside of the ebike?

At present I can take the battery bay lid off and charge each one individually using Anderson plugs, but I was hoping to stretch the wiring so I can place 5 pairs of Anderson clips on the outside of the bike and charge them from there, while not having to change any of the series wiring inside that is leading to the controller.

The batteries ive ordered are due in the next week or so with chargers, I'm pretty sure they are plastic outers with a single positive and negative coming off to connect up with the charger/appliance.

Thanks
Paul

 

[Waylander]

Hey uncky Scrouge, thanks for the advice, Ive come across that one before. Its a BMS system right? as to say a little board on the battery that ensures the recharge voltage is spread to all the cells evenly? so some cells don't undercharge and stuff? The company supplying the batteries in china said they come with BMS and the chargers are designed to the batteries, so im hoping with each battery pack being charged by its own 12v charger then they should stay pretty well balanced.

Apparently the battery is rated 3c for its peak amperage. As each battery is 12v30ah then can I use that to calculate how many peak amps each battery will provide?

 

[MrAl]

Hi,

Yeah a 12v Li-ion pack is not at all like a 12v lead acid battery. The 12v Li-ion is actually made from three cells in series.

Charging the pack with a 12v charger means that the three cells get charged in series so they all must have the same current. The problem is, one might charge all the way up sooner than the other ones and then it's terminal voltage will rise above the maximum for Li-ion. This will cause a big problem and maybe even fire.

A "series" charger will monitor all three cells, and provide a secondary current path AROUND the cell that charges up first. That allows the other cells to charge to their required max while the first one just idles. This prevents overcharging any one cell.

To charge these yourself without the benefit of a charge "balancer" (as they are often called), you would have to monitor each cell individually and either disconnect the cells that are already charged up or else just stop charging and allow some cells to not charge all the way. Alternately, you might find a charge balancer for a three cell pack. Unfortunately, this kind of circuit needs access to ALL the terminals of EACH cell inside the pack, meaning there would be four wires connected to each pack, not just two.

 

[ClydeCrashKop]

The company supplying the batteries in china said they come with BMS and the chargers are designed to the batteries.

If you are sure that the outputs are completely isolated from the mains input, (they probably are) then your circuit is correct. Go for it.

 

[Pommie]

Plug in two of the chargers and measure the voltage from the positive of one charger to the negative of the other, if it is zero then you're good to go. Any reading above mVs is a problem.

Mike.

 

[BobW]

I would think that a 3-cell series connected Li-ion pack, with only two external connections, would have to contain internal circuitry to ensure balanced charging. Otherwise, it would be unusable.

 

[Waylander]

If you are sure that the outputs are completely isolated from the mains input, (they probably are) then your circuit is correct. Go for it.

Cheers Clyde, yeah the inputs on the plugs should be isolated on each of the chargers from the outputs to the batteries

Plug in two of the chargers and measure the voltage from the positive of one charger to the negative of the other, if it is zero then you're good to go. Any reading above mVs is a problem.

Mike.

Cheers Mike, this is a great way to test to see if they are bridging, as you say if I test two chargers on two of them and see if the voltage on one is higher than when its on its own it should confirm if its viable or not.

And to Mr Al, thank you, but I think I should be ok in balancing as the batteries come with bms and each charger should act as its own series charger for each battery pack. That way they should stay relatively balanced right?

Thanks for all the help on this post, once ive done the testing I will report back here and confirm my findings.

With a bit of luck the extra amperage from the new li-ions wont fry my ebike e-controller

Wish me luck

 

[MrAl]

Hi,

There is no such thing as "relatively balanced". Either the controller can balance them or it can not.
Sorry for the bad news but Li-ion cells are a very different kind of battery that needs close attention to some important details about charging and using.

 

[Waylander]

Hey Mr Al, yeah they seem a little more tricky than the old sla's. Is the controller the BMS board? or is that in the charger instead? or something different altogether?

I'm more of a programmer than an electrician Cheers

 

[Pommie]

When I said about testing between two chargers I meant without any batteries connected.

I'm assuming the chargers are intended for a single 12V battery and the balancing is taken care of inside the batteries.

Mike.

 

[Waylander]

Hey Mike, yeah each 12v battery has its own 12v charger to charge it independently. I presumed that the bms's would be in with each of the batteries and that they would be smart enough to balance out the cells using the corresponding chargers I have ordered with them from china. (Must admit cant confirm till they arrive in the next week or so) I'm basically stringing up 5 batteries in series with the positive and negative coming off the 60v30ah battery array which connect up with the e-bike's controller.

I was hoping to just add an extra wire coming off each of the battery terminals after setting up the 60v wiring, one off each of the positives and one off each of the negative wires on the batteries, so I can then fit them with anderson clips which can fit in a recharge point on the outside of the bike, so I can plug the 5 chargers in to the 5 sets of Anderson clips there.

Before I removed the slas, I was charging 4 x 12v batteries in series with a 48v charger and charging the 5th battery with its own 12v charger. I could take the battery bay off to recharge each of these new batteries individually by disconnecting the 60v series circuit, but im hoping to lock my batteries in place with metal bars, as the kids will liberate them around here if I sneeze

Much appreciated Mike, but I'm a little confused as to checking to see if the chargers are bridging if there are no batteries in the circuit. Could you clarify please mate.

Believe it or not this sounded all sweet in my head when I was ordering them, how unexpected questions stab the soul

 

[Waylander]

Here is the 60v circuit without the wires leading from the batteries to the chargers: **broken link removed**
and heres the 60v circuit with the wires coming off the batteries to the charger: **broken link removed**

Im just not sure if each of the chargers in the second link will bridge with each other and fry stuff due to the 60v wiring. I was hoping if I put a switch in the 60v wiring to the controller and then charge the batteries with the 60v circuit broken, then may be they wouldn't bridge and they would just charge the appropriate battery

 

[MrAl]

Hey Mr Al, yeah they seem a little more tricky than the old sla's. Is the controller the BMS board? or is that in the charger instead? or something different altogether?

I'm more of a programmer than an electrician Cheers

Hi,

It should not be hard to tell if a given charger can not only charge a series wired Li-ion pack but also balance it. The tell is how many wires it takes to connect the 'charger' to the cell pack. For a given number of cells N, the number of wires should be N+1.
So for one cell there would be 2 wires, for two cells three wires, and for three cells there would be four wires that connect the 'charger' to the pack. If there are any less than that then the charger does not balance the cells. This of course excludes specially designed packs that have built in charger controllers.

There is a secondary mechanism that comes into play too however. That is any built in protection of the cells used to build the pack. If they have a protection mechanism built in then they probably stop charging after a certain point, but then the other cells dont get charged enough. So maybe the pack wont blow up, but it probably wont charge correctly either.

 

[Waylander]

Hi,

It should not be hard to tell if a given charger can not only charge a series wired Li-ion pack but also balance it. The tell is how many wires it takes to connect the 'charger' to the cell pack. For a given number of cells N, the number of wires should be N+1.
So for one cell there would be 2 wires, for two cells three wires, and for three cells there would be four wires that connect the 'charger' to the pack. If there are any less than that then the charger does not balance the cells. This of course excludes specially designed packs that have built in charger controllers.

There is a secondary mechanism that comes into play too however. That is any built in protection of the cells used to build the pack. If they have a protection mechanism built in then they probably stop charging after a certain point, but then the other cells dont get charged enough. So maybe the pack wont blow up, but it probably wont charge correctly either.

Hey Al, cheers that's honestly really useful. I saw a video a while back on the balancing but couldn't get my head on it, thats just cleared it up. Not to mention the fact that I didn't consider that the board on the batteries might just deal with over charge issues rather than providing the balancing. The makers are aware its for an ebike project so I will be interested to see what arrives at my door.
Finding decent capacity batteries with the correct dimensions to fit on my bike so I don't have to start chopping it up took quite a while. It would suck if I needed to access the battery inners to wire them up to balance them. With the best of luck I think I can hope for there will be a half dozen wires leading off from each battery to each charger.

It certainly makes the circuit a little more tricky when it comes to wiring up the 60v30ah array and stretching wiring to the outside of the bike to charge them, that would be a hell of a lot of Anderson clips

A lot to ponder, but a mega thanks for the assistance.

 

[MrAl]

Hi,

Yeah, but for something like this maybe the charger would be build into the pack or close to the pack so that it connected to all the cells but you only need two wires to connect to the main source of power. The controller could work off of DC, then you supply DC to the controller and the controller is either built into the pack or mounted in the same place as the pack so the multiple wires never have to be dealt with once the pack (and charger) is done being built. Just two wires to plug in somewhere.
The whole charger doesnt have to fit near the pack, just the balancer or maybe the whole circuit while the DC source (transformer, rectifiers, regulator) can be mounted separately in the garage or something. That would still be just 2 wires to deal with when it came time to charge up. With a connector it would just be a matter of plugging it into the charger/balancer circuit and turning on the power supply.

 

[Pommie]

Much appreciated Mike, but I'm a little confused as to checking to see if the chargers are bridging if there are no batteries in the circuit. Could you clarify please mate.

If the chargers aren't isolated then the neutrals are probably connected, if this is the case then measuring from the positive of one charger to the negative of another should read 12V.

Mike.

 

[alec_t]

A pic to show what Pommie is suggesting: