Charging Two Packs In Series

[The Journey Guy]

I'm attempting to do a schematic of the way I'm going to setup my new bike build, using 2 LifePo4 packs, each rated at 24v 20ah in series, for a total of 48v 20ah.

I'm doing alright, except where it comes to the charging aspect of this setup. What is the preferred way of charging two packs that are in series?

Would it be best for me to place an On/Off switch inline on the Neg side of each pack, so that I can electrically remove each pack from the circuit for charging with two separate chargers?

I do not have a 48v charger, only two 24v chargers, so this is why I ask the question.

Thanks for any ideas and suggestions!

 

[dogman dan]

Use the two chargers at once. You will be able to leave em connected in series. Each pack will charge seperate when series connected. Paralell would be different if no diodes were used. It will charge twice as fast as one charger of the same amps at 48v.

If you have seperate wires for charging, you are already all set.

 

[The Journey Guy]

Use the two chargers at once. You will be able to leave em connected in series. Each pack will charge seperate when series connected. Paralell would be different if no diodes were used. It will charge twice as fast as one charger of the same amps at 48v.

If you have seperate wires for charging, you are already all set.

Yes, the packs already have the BMS and charger wires, as came from the factory. I have been using them singley for some time now, but with the new motor, I will be placing them in series.

I am including two 100v 100a diodes in the circuit because of what I have learned on this forum. But I still don't understand why I can use both chargers at once, and leave the packs in series.

With the packs in series, the controller sees 48v. So, with the packs still in series, why doesn't the chargers see the same 48v? I don't understand.

 

[The Journey Guy]

Can anyone explain all this to me? I've been reading and doing searches, and I'm totally confused. Dogman says it's alright to use my two chargers without removing the packs from the circuit. In doing my searches, I came across this diagram that seems to contradict his statement. It looks like the attached file below, except of course I'm using two 24v packs instead of the 12v packs shown.


That's why I asked earlier how a 24v charger can charge a total voltage of 48v resulting from a series connection. Would someone try to explain this to me? Thanks.

 

[scoot]

I'll try I guess... just hope I don't make it worse

I don't see a contradiction in the diagram.

Let's say you have both packs all connected up in your whole system like for normal operation, except that you disconnect between the two packs. Then hook up each pack to a separate charger (voltage matched of course). that sounds safe right?

Then by simply reconnecting the two packs back together serially, neither charger will see a voltage difference between them. To confirm, simply measure the voltage between the packs with a volt meter BEFORE reconnecting them serially ( with both chargers running). Hope that helps :wink:

 

[vanilla ice]

DM is right, you are still just charging two 24v packs. The fact that there is a single wire going between the two packs is of no consequence for the chargers.. Well as long as you have isolated chargers, which most are. If not, that is the only case you would need the switch.

Like scoot says, if there is any confusion hit up the charge port with a DMM and make sure you aren't plugging a 24v charger into 48v..

 

[The Journey Guy]

Thanks scoot and VI, I believe I can wrap my mind around that explanation. I can certainly understand the concept of measuring the voltage at the charging plug while the two packs are still in series to see what voltage the charger will see.

Many thanks to you all in this thread for your time!

 

[dogman dan]

Yeah, it's hard for us not so electronicly gifted guys to visualize. But each charger will only see the 24v, since the charger wires only see that half of the entire string. I had to just take it on faith for quite awhile, but the light bulb went off for me when I undersood the basic difference between series and paralell connections. A paralell string lets one cell charge the adjacent ones, but a series string does not. I doubt I could explain why even now. I just don't have the education to know the why, just the way it is. In my dogbrain it sorta makes sense, a paralell string, with all the + connected to one wire and all the - to the other is really just one battery so charge one cell, the current leaks to the others. Logically a series string, connected different would act different, just don't ask me why. But where the wires exit the whole string makes the difference. Connect at one end, and at the middle, get 24v, connect at one end and at the other end, and you get 48v.

Similarly, by connecting a charger to the right cells in the right place, you could charge the pack one paralell 3.5v string of cells at a time. This is usually step one of trying to resurect a pack with some out of balance cells. But since one cell in a series string won't charge the next one, you can charge a 48v pack, one 3.5v cell at a time.

Both chargers at once, It would be a good idea to break the connection between the packs to charge. I again don't understand the isolated , not isolated issue for running both chargers, and I forgot about that issue. When I charge my two 24v packs that I use as a 48, I only have the one 24v charger, so I can charge both, and don't need to separate the packs to charge.

 

[The Journey Guy]

Both chargers at once, It would be a good idea to break the connection between the packs to charge. I again don't understand the isolated , not isolated issue for running both chargers, and I forgot about that issue.

Ah man, say it isn't so!! Just when I thought I had this understood, now this... OK, now that this new wrinkle has come up, how do I find out if my chargers are isolated or not? Geez, this is getting more complicated than I thought it would be....

 

[dogman dan]

Easy enough to disconnect the packs to charge, or install a switch between the two. It's just the wire that connects the + of the first one to the - of the second that needs to be disconnected. You can ask your charger vendor if they are isolated and if they are, no worries.

 

[Rassy]

Somebody once gave us a way to determine if the chargers are "isolated". If I understand it properly, it is the third wire of the AC plug that may tie the chargers together, and I am not sure why, but some have suggested a charger with just a two prong plug will be isolated from other chargers. If you can measure the output voltage of the charger when it is plugged in but not attached to a battery (most chargers you can, but some will not come "alive" until they sense a battery is hooked up), just plug in both battery chargers and use your voltage meter with the + on one charger's positive lead and the - on the other charger's negative lead. If you get a voltage reading they are not isolated. If you get no voltage reading they are isolated. Then check again that you get a voltage reading from the output lead of each single charger. If you still feel uneasy, put a small fuse in each positive lead.

As mentioned earlier in this thread, some people here use multiple isolated single cell chargers to balance their packs without disconnecting the series wires between cells.

Good luck.

 

[amberwolf]

You can test for isolation with a multimeter.
--Disconnect the charger from batteries and wall power.
--Set meter to ohms, and connect the black lead to the negative output of the charger.
--Touch the red lead to each of the wall plug's pins in turn, noting the readings.
--Move the black lead to the positive output of the charger and do step three again.
--Each reading should be infinite, or very very high (megohms). Any low reading means they're not isolated.

As long as they are isolated, you can connect them like this:
View attachment 24V chargers for 48V pack.PNG

 

[The Journey Guy]

Thanks Rassy and amberwolf! I did what each of you suggested, and the chargers both passed the isolation test. So, it looks like I'm good to go with just leaving the packs attached in series, then using each pack's charging port as normal, and charging them both at the same time.

Thanks very much for taking the time and effort to share the information!

 

[amberwolf]

No problem. This is what I am about to set up on my own bike, once I complete either one of two modifications. (One is the suspension system, which will then leave me room for a fourth battery at the rear, and the other is a two-battery carrier that would ride on the top tube just behind the headstock at the very front, adding one battery and moving the rearmost existing third battery up there too).

There is a third option I am working on the idea for as well, which is to use a single 24V charger to automatically switch between two pairs of 12V SLAs. In overnight mode it would simply charge the first pair until the voltage rises to full charge or the current drops below a specified value, then a relay will switch it to the other pair. When it reaches the same point, the relay will drop out and it will go back to the first, and continue alternating like that to keep them topped off. The chargers I use are 3-stage wheelchair/powerchair chargers, designed to stay plugged in if the chair is not in use, as far as I can determine. In either case, it wouldn't be plugged in longer than a whole day, as I am going somewhere nearly every day.

In catch-as-catch-can mode, it would charge for 30 minutes on the first pair, switch to the second for 30 minutes, etc., in default settings. The time can be altered to as little as 10 minutes, in case I know I am going to be plugged in for much less time, so that each pair gets at least some charge.

The only reason to design it this way at all was because I had only one charger until a few days ago, and having started the idea I'd like to finish it. It would also be a couple of pounds lighter with only one charger. Since one charger is a 3A and the other a 4A, I'd use the 4A charger for this option.

I considered making a series/parallel switch for the packs, so I could charge as 2S/2P which would be faster and electronically simpler, but that requires a heavy-duty switch or contactor (relay) as well as more heavy-duty wiring, as the motor power will have to run thru the switching device, too. Doing it the other way only requires maybe 14gauge wire to handle the charging current, and the relay can be pretty tiny too.

 

[The Journey Guy]

Sounds like a great setup. I like the idea that it will switch from one bank of batteries to another by itself. Very nice!!

I assume you will make a thread on it, yes? I'd like to follow that one.

 

[amberwolf]

Eventually, once I have something worked out, I may make a separate thread for it. If not, it will at least be on my bike project blog in my signature.