battery isolation for portable power.

Iv been trying to wrap my head around what im trying to do and cant quite figure it out.

Pretty much i have salvaged 8 18650 size cells that i would like to run in parallel to make a portable battery. In order to boost capacity, i would like to buy new cells and make another pack that is parallel also. I want them parallel so its a nominal 3.7v system that i can boost to 5v for USB devices

I know its bad to run old and new batteries in parallel do to various reasons such as self discharge. My idea is have 2 usb ports for one pack and 2 ports for the other pack and have an input/output protection PCB per pack. Now to keep this more portable i only want to use 1 charger and this is what confuses me. I can charge each pack separately, but that would require me to swap packs on the charger, and i would rather not have to worry about doing that.

Long story short, how can i charge 2 battery packs at the same time but keep them isolated from each other? Diodes maybe? Also with how the protection circuits work having the charge leads connected to the output leads and the batteries on there own leads, when the charger is outputting over one amps, will it hurt any device plugged into it or will it regulate the current down to one amp with no harm done? Link to usb charger and protection circuit.

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-max-2 years ago

Charging multiple batteries of the same nominal voltage ratings in parrelell shouldn't be an issue. The voltage between the 2 packs will always be the same once they are connected together, however, you want to make sure the voltages are the same when you do connect them together. If one was partially discharged, at 3.7V, while the other was topped aff @ 4.2V, there will be LOTS of current coming from the charged pack, going into the other one. With small voltage differences, the current is small, and thats not a big deal, and it naturally balances the 2. However, with bigger V differences, damaging levels of current can flow.


Once they are connected together like that safely, however, it becomes effectively one larger capacity battery. The exact charge/discharge curve might be differently shaped, as it will be the sum of the 2 pack separately, but no one usually worries about that unless it is a critical application where everything matters.


Even if the capacities of the batteries was wayyy different, but same type and same voltages, then it does not matter! The charge will always ballence between the 2, and the bigger battery will do more of the work, and the work both batteries do is proportional to their capacity/size.


However, batteries with different charge/discharge characteristics should not be connected together, like LiPo and NiMh. They have different fully charged voltages, so either charge it up to max out the NiMh battery, and the LiPo will be way too low on voltage, and probably be damaged from undercharging, or charge it up to 4.2V and kill the NiMh. Either one is a good idea, in fact it will probably lead to a fire. Don't do that, even with less extream examples.

Ksp3cialK (author)  -max-2 years ago

This makes much more sense! I failed to think about connecting the 2 packs together to charge at different voltages. So this answers my initial question, it seems the only safe way to use the packs separately is to charge them separately as well to not have big amounts of current flow from one to the other. Thank you!

-max- Ksp3cialK2 years ago

No problem! Also, if you are looking into making that charger, I would look into getting a small 3S LiPo battery, which is 9V-12.6V, and powering a car cigarette lighter phone charger with it. The center pin on the cigarette lighter input portion of it is positive, and the outer contacts is negative.


Those car phone chargers have a high efficiency SMPS buck converter inside them, and are around 80% efficient, but actual efficiency depends on input voltage, output current, frequency of operation, the quality of the parts inside, etc etc etc, the alignment of the planets, the amount of time since the big bang... the list goes on and on :)

-max- -max-2 years ago

The video is showing a weak 9V battery, (the wrong type in the actual video, do not use a heavy duty one.) Just use the 3S LiPo battery instead! Note: Since a 3S LiPo battery is 3 cells in series, you will need a special LiPo charger that has a balancing port already on it to stay safe. It will keep the cells voltage stable and prevent overcharging to any particular cell. For discharging it, it really does not make that much difference. Just be sure to not discharge the pack below 9-10V (3V/cell)

-max-2 years ago

But as others said, it is a good idea to measure the tempurature of the cells in the pack to make sure they do not overheat due to practical issues outside the basic theory.

-max-2 years ago

The protection circuit is a good idea, and you could certainly use a few separate charging chips with their own isolated power supply if they do not already have one of those. (the USB one probably does not have an isolated output from the USB.)

iceng2 years ago

Charging lion cells in parallel works if you can stop charging any single battery that gets too hot, meaning charging it further will damage it and possibly start a fire.

Ksp3cialK (author)  iceng2 years ago

what would cause any certain cell to get to hot if charging voltage does not exceed 4.2v and charge current is less then .5C? Just a defect/short inside the battery?

Iv read about these batteries quite a bit, but still have a lot to learn. thank you for the reply

iceng Ksp3cialK2 years ago

Several cells in parallel,

but no guarantee they will share the charge current equally.

Usually one cell hogs the current and reaches full charge first.

The other 7 cells overpower the one and hold the voltage below the cut off regulator voltage because they are not fully charged, but current is still flowing to the full charged cell which soon can only turn that current into throw away heat and get damaged ever so slightly due to over charge heating.

Repeated cycles damage the cell further until it does something like venting internal chemicals it should keep and you soon end up with a dead cell.

It is painful to the eye to read a worm like that, a bit time to format the text would be a great idea.

It also seems you lack the basic understanding of charging and battery use, no offence!

Whenever you connect batteries in parallel you will end up with trouble sooner or later as there is no way to prevent self discharge through other cells, in severy cases (especially with Litum based batteries) overheating and fire.

Same for the charging, the charger has no way of telling when a pack is full, that is why you always connect a single battery pack per charging channel.

If you want true portable power check the Kraftwerk project on Kickstarter, USB power from butane gas (lighter refills).

Ksp3cialK (author)  Downunder35m2 years ago

Sorry about that, I was in a rush typing. Iv searched for hours on this and found nothing on what Im specifically trying to do so I figured I would ask a question about it.

I have seen some schematics that use diodes to isolate batteries. I have also seen many topics on the RC forms where people will parallel charge there lithium based batteries with success.

From what I understand is batteries in parallel will only self discharge if there is one or more bad cells due the the healthy cells trying to charge the bad one. This is the reason Im trying to isolate the old and the new battery pack to keep this from happening. And with the charging, a smart charger for li-ion cells will stop once it reaches 4.2V. That is where the protection circuit for the battery will also kick in preventing more power from entering one of the battery packs if it reaches a full charge first, thus current gets transferred to the other battery until it reaches the 4.2v mark and the charger shuts off.

As far as portable, I looked up the kick starter and yes the design is cool but I don't want to spend $100+ for portable power that I have to buy refills for. If I spend that kind of money, it will be on a solar panel.