Charge a battery with another one

Hi !

We have a bike with a frame integrated battery, but the capacity is a bit weak (250 Wh). I'd like to build a battery that would be able to feed the other one by the charger plug. Do you know if there is an existing electronic module that would allow that?

Thanks in advance

Didier

The problem with that approach, is if you connect a full one to an empty one, the current rushes to the empty one too fast. This is bad for both batteries cells, and on the input side of the empty one, it can't handle near that much current.

What you need is to connect both batteries, if similar enough, in parallel. The connection is made on the output plug, using a y shaped connection adapter you make yourself. If you have to, add a new plug to the bikes wiring harness, between the battery and the controller.

ALWAYS, the packs are charged disconnected, then connected only when full. This will make your battery big enough to do what you want. They don't need to be the same size, just the same type of cells, and same voltage. One won't charge the other enough to matter, They will mostly just discharge together, with the larger battery discharging more of the current than the smaller one. In other words, the one with the highest voltage at that moment will provide the most current.

Its easy, and works fine. Its not that much hassle to disconnect and charge separate, and no damage to either battery will happen.

you need a DC-DC converter. but you lose 20% in power losses.

its way more efficient to just hook them up both and drive with both batteries at the same time.

Actually, the idea is to build packs we could sell to the customers who buy this kind of bikes, it's not for personal use. So I really need a plug and play solution, I won't have acces to the bikes I sell packs for. So my "challenge" is just to have a second battery that I I connect by the charger socket...
I've heard about the DC-DC converter. What would be the most efficient solution? Boost or buck? I don't know those very well...

What pack voltage are you trying to charge?

There are plenty of dc-dc units out there. Some have adjustable current limiting as well as voltage adjustment, both of which you would need. The charging input on many packs often has a fairly low current rating, so you would want to keep below this. The cheap dc-dc converters often have wildly exaggerated output specifications, so if you want one to actually work, pick one with about 2-3x higher rating than you will actually use.

These come in both buck and boost flavors. By using a boost converter, your source pack could be a lower voltage. In general, the closer the voltages of the two packs, the more efficient the dc-dc converter will be.

I agree with the others that point out this is not an ideal way to do things but if there is a real need, I don't see why it won't work.

Diva said:

Actually, the idea is to build packs we could sell to the customers who buy this kind of bikes, it's not for personal use. So I really need a plug and play solution, I won't have acces to the bikes I sell packs for. So my "challenge" is just to have a second battery that I I connect by the charger socket...

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See above. Not hard; just make sure they are the same chemistry, configuration (i.e. both are 14S or whatever) and are both charged.

I've heard about the DC-DC converter. What would be the most efficient solution? Boost or buck? I don't know those very well...

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You don't need a DC/DC converter.

You could connect both the discharge and charge ports in parallel using a "Y" cable for each. That way they will discharge together, and charge together. You need packs of matching voltage for this to work, but I don't see any disadvantage to this arrangement aside from a bit of extra wiring. Both packs remain at the same voltage all the time, and things don't need to come apart for charging.

billvon said:

You don't need a DC/DC converter.

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Since hes talking about selling packs to customers, and it is probable that many or even most of those are unlikely to be able (or willing) to test battery voltages each time to ensure they are the same, or even just to make sure they are actually both fully charged, before plugging one into the other, itd be much safer to have something that will limit the current between them if there is a large disparity in voltage for whatever reason.

Whether thats a DC-DC or soemthing else, I couldnt say, but if I were doing this I would definitely use something as foolproof as possible.

amberwolf said:

Whether thats a DC-DC or soemthing else, I couldnt say, but if I were doing this I would definitely use something as foolproof as possible.

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I agree. It would be bad if a customer tried to connect two packs that were at drastically different states of charge. With the right dc-dc converter, the current limiting function will prevent smoke and make it fairly idiot resistant. But I also agree that this is not the ideal way to combine two packs as the losses in the dc-dc are significant and the converters may be prone to failing.

amberwolf said:

Since hes talking about selling packs to customers, and it is probable that many or even most of those are unlikely to be able (or willing) to test battery voltages each time to ensure they are the same, or even just to make sure they are actually both fully charged, before plugging one into the other, itd be much safer to have something that will limit the current between them if there is a large disparity in voltage for whatever reason.Whether thats a DC-DC or soemthing else, I couldnt say, but if I were doing this I would definitely use something as foolproof as possible.

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If it were me I think I'd just use a current limiter. A lot simpler and higher power (once voltages were equalized.)

Endless-sphere members seem to have a strong disinclination to diode based pack isolation, but I have this on both of my double pack bikes - one a Schottkey from when ebikes.ca used to sell them, and one ideal diode from Sphere member "Tiberius" in England. I never have to think no matter what the state of charge of either battery, or worry that they will become differentially charged so that a big voltage gap causes one to charge the other. In my opinion, the peace of mind this gives you is worth the tiny losses involved through the diode. I know you can always pay attention to equalizing the charge state of the two packs - I don't want to bother worrying. Why exactly diodes are frowned on is beyond me for two packs discharging in parallel.Why not bulletproof?

The main issue with isolation diodes is their heat dissipation. For something that draws 20 amps, for example, the diode will be dropping nearly 1v and dissipate 20W of heat. With a really good Schottky diode, you might cut it in half. So as long as you have a large enough heat sink and it is located where it gets good cooling, this will work fine. For some monster 100A system, it becomes impractical.

There is an "ideal diode" circuit that uses a FET to turn on only when current is flowing one direction. This greatly reduces the voltage drop and heat. The circuit adds a lot of complexity and draws power though.

It also wont help if youre doing what the OP wants, which is to plug a second battery into the charge port of the first.

The only thing you can tell customers then, is to buy two identical packs, run one pack, unplug, then run the second pack.

Nothing else will work for them, aside from selling them a bigger battery in the first place. This is the policy EBK uses, because you just can't trust a customer to follow the procedure for paralleling, and then you end up liable.

Here at the sphere, I could tell the same customer whatever. But on that EBK chat, absolutely not. Run one, then unplug and run the other is all I could tell them on the company website or phone.

Diva said:

Hi !

We have a bike with a frame integrated battery, but the capacity is a bit weak (250 Wh). I'd like to build a battery that would be able to feed the other one by the charger plug. Do you know if there is an existing electronic module that would allow that?

Thanks in advance

Didier

Click to expand...

Sometimes we try and solve a problem with a work around that only masks the real issue and maybe creates more and different issues, this is not a good approach with customers and batteries.

Maybe this effort would be better used in designing a bigger capacity battery, better cells and more of them will give a simple choice for customers to buy and no safety issues for you,