The main device is a bank of Li-Po 18650 batteries as the main power source. Sometimes the device will be plugged into AC power when needed for longer periods of time than what the battery packs will handle. Sometimes the device will not be plugged into AC power and will only use the battery packs as the power source. If connected to AC power and there is a power loss, the device should switch to the battery pack for power (UPS). The main device will power 2 other devices. One will be 5V @ approximately 800 milliamps. The second device will be 12V @ approximately 1 amp.


  1. What charging options are available for this many batteries in this configuration?

  2. Is there a charging option that will keep the batteries balanced in this configuration?

  3. Should I be using batteries with protection IC?

  4. I found this board that sounds like it may work, but is there something similar that is cheaper or is there a better board for my setup? Open UPS

  5. Is my layout ok? If not, what should I change?

Thanks!enter image description here


closed as off-topic by Leon Heller, Ricardo, Daniel Grillo, brhans, pjc50 Jun 16 '15 at 9:16

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  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Leon Heller, Ricardo, Daniel Grillo
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  • \$\begingroup\$ Li batteries tend to die permanently if they are under-discharged (under ~2.5v/cell) - how to you intend to prevent that from happening? \$\endgroup\$ – user2813274 Jun 12 '15 at 21:45
  • \$\begingroup\$ However they need to be used, we can do that. If they need to be disharged regularly at a voltage less than 2.5v, we will not charge them until necessary. For the setup where they will be connected to AC, it will only be for about 1 week at the time. \$\endgroup\$ – user1618156 Jun 13 '15 at 0:59

Check the battery manufacturers charging specification data sheet for the details on the chemistry / battery type you want to use.

Letting the batteries discharge too quickly or letting them discharge to a low voltage can permanently damage them. Also the charging rate needs to be within the manufacturers specification or you will damage the chemistry / life.

The batteries tend to self balance themselves, however again most manufacturers don't recommend using multiple batteries in parallel.

That board looks cool, and it probably will save a lot of work, and end up costing about the same as diy.


1) Given that you're using regulators to power your downstream system, you could get away with simply connecting a Li-Po-compatible battery charger to your +ve & -ve buses, because the charger isn't going to apply any more than 4.2V per cell-in-series (i.e. 8.4 Volts). But bear in mind that when recharging, the Li-Po charger will need to be rated for the power demand of both charging (at the rate you choose) AND the load (assuming you want the device to work while charging).

2) Certainly. Many options. Do the math on the max recharge current needed to recharge this behemoth of yours within the time period you need. Google. Just know that putting 9 cells in parallel is non-trivial, make sure you initially connect them together in a fully charged state, and to use BPMs on each cell. Having 2 such batteries in series to get you 8.4V (full, down to 6.0V flat) suggests 'cell balancing' would be ideal, but whether you can find a charger within budget that will do balancing at whatever your system current draw is, can't be gleaned from your question.

3) *Always (*almost). Li-Po are fickle and deadly mistresses. Anything above 4.25V (check datasheets for your particular 18650s), anything below ~3.0V (check datasheets), current's in excess of their stated Maximum-xC-discharge-rate, will shorten the life of the cells well beyond reasonable expectations. That's why cell/battery protection modules/PCBs are typically added to 'loose' Li-Pos, to protect hobbyists from what most of them don't understand.

4) It's nice, but its 'power path' feature probably isn't needed in your application, and if it's too expensive, then look for simpler recharge-only alternatives, which comes back to Question-2.

5) as someone commented above, you also want 'under-voltage lock out' (UVLO) between battery & voltage-regulators (or Vregs with settable UVLO threshold voltage), to stop your system bleeding the battery dry below the safe level of ~3.0V/cell.

It's hard to be specific or more helpful without (a) knowing a lot more about what's being powered & for how long, & (b) finding someone else willing to engineer this for you properly.


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