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I want to make a lithium ion power bank. I have lots of lithium batteries but I don't have very many that are the same. So if I connect 20 different ones in a parallel circuit and they all have the same voltage ,do you have to charge it with the lowest voltage/current value of the lowest cell? Or would I be better off connecting them in sequence? If sequence is better could someone recommend a charger for me that will go up to 20-50v 1-10amp....just as a side question for charging, can I use a c2h( can't remember the name ) charger to charge lithium ion

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Any time that you connect batteries in parallel, you generate a system where one charges the other or one discharges the other. Unless the cells are very tightly matched, this does not work well.

Connecting them in series for a higher voltage pack is possible, but unless the cells are matched very well and of a chemistry that allows this without extra circuitry, you will need to balance.

The saying "a chain is only as strong as its weakest link" applies to a mixed combination of lithium cells. You can only source as much power until your smallest cell is drained. Then you start to destroy it.

Lithium batteries are highly dangerous. The can catch fire and burn very rapidly when charged improperly. Just by asking this question it is highly suggestive that you may not have the knowledge to put something like this together safely. I would never attempt it, because there is no benefit over using matched cells.

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  • \$\begingroup\$ Thanks Joe. And I already new it but I was just making sure I understood it correctly and I wasn't sure if u could do it with lithium batteries. I have only done it with capacitors \$\endgroup\$ – user100476 May 26 '16 at 13:59
  • \$\begingroup\$ Even with capacitors (when storing energy) you often have to have balancing circuitry. This is because super capacitors have a maximum dielectric voltage. So you need to bleed voltages down the line to make sure each charges up equally. Similar idea as balancing packs. \$\endgroup\$ – Joe May 26 '16 at 16:03
  • \$\begingroup\$ Ok, I might go back to my original plan of making a capacitor power bank. Just curious how much is a super capacitor that's about 3v 10F ? And were would you get a good deal on them? All of them I have say are like 10$ \$\endgroup\$ – user100476 May 26 '16 at 17:14
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The configuration you intend to construct depends highly on what you are trying to power. Since lithium-ion batteries are 3.7 volts nominal, you're likely going to need to put them in series to achieve a higher voltage. It's simplest to charge them when in parallel versus series. A digital power supply set to 4.2 volts would work, but you should never leave them alone. Prior to any of this, each cell should be checked for internal shorts, especially if you're trying to reuse laptop 18650 cells. If they don't hold at 4.1 volts or higher for a few weeks then they are suspect.

If you do put them in series, they will require a battery management system (BMS) that is built for a specific voltage for balance charging. A 4-series bank would require a 4S BMS (16.8V). Once you build in series, you can then add parallel banks as well, and at this point you're adding capacity to the bank. Those parallel banks are 'self balancing'.

your mention of a capacitor bank-- keep in mind capacitors have very low energy density so you might reconsider that.

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