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I have a project in mind based around a Raspberry Pi Zero. The case I am using has slots for two AA batteries so I thought I would use some of the AA sized Li-ion batteries.

The batteries I want to use are 3.7V 2300mAh.

The charge circuit I found is: http://www.banggood.com/37V-Liion-Battery-Mini-USB-To-USB-A-Power-Apply-Module-p-928948.html

Ideally I would like to have two batteries in parallel so I could get 4600mAh, but I know there can be issues charging and discharging in parallel, plus I have no idea if that charge controller will do that.

My next option is to just use a single battery with that charge controller, that seems the easiest. Although I cant actually find any reliable info saying this circuit can charge Li-ions, Can I use anything advertised as charging Li-po on Li-ion?

One other option would be to put the two batteries in series and an LM7805 to bring the voltage down to 5V, would this be suitable? I would use an external charger that way.

Sorry for the broad question, if I use google all I get are things for Li-po batteries, and if I look for Li-ion charge modules all I get are the external type.

I am pretty good with amateur electronics, but I've never worked with these before and I don't want to get it wrong.

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    \$\begingroup\$ That battery capacity sounds pretty suspicious. I have not looked at AA sized Lithium ion batteries, but that seems like a very high capacity for that form-factor. Which makes me think the batteries are possibly being sold by a shady vendor and may be of poor quality. \$\endgroup\$ – mkeith Jul 24 '16 at 17:58
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    \$\begingroup\$ You should have seen the 9900mAh ones for 99p. These might be a bit dodgy but I saw several battery reviewers showing that these capacities are possible, I thought it was worth a gamble, worst case I'll just buy some smaller/better ones. \$\endgroup\$ – Owen Tourlamain Jul 24 '16 at 18:03
  • \$\begingroup\$ I admit to being a bit unsure. If you have found reviews, and you are reasonably comfortable with it, then go for it. Another option is to use one lithium ion cell and a dummy cell in series. The dummy cell is conductive, so the system only sees one battery. These are intended for devices which use 2AA batteries. They use one lithium battery and one dummy battery and the device will be reasonably happy with it (depending on the device... the cutoff may be a bit low for the lithium battery, and may shorten its useful life). \$\endgroup\$ – mkeith Jul 24 '16 at 18:30
  • \$\begingroup\$ There is a good chance you can get away with two in parallel, also. Just make sure there is some type of over-current protection for EACH cell. This can be a fuse or ptc or active protection circuit. It should be rated for 9V or more, to be safe. Also, at the moment when you put the cells in parallel, make sure they are at the same voltage (within, say, 10 mV). Otherwise a large equalization current may flow until they balance out. \$\endgroup\$ – mkeith Jul 24 '16 at 18:34
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You are asking about the different ways to charge a 2S1P (two batteries are connected in series) and a 2S2P (two batteries are connected in parallel) battery pack. Use these terms when researching this subject.

The better battery chargers will balance the charge over all the cells in a pack. And will tend to charge the batteries faster near their charge rate limit. They should also stop charging when the batteries are full. Detecting the end of charge depends upon the battery chemistry and manufacture's recommendations.

The cheaper battery chargers will not bother with balance and will (should) charge batteries at a fraction of the charge rate limit. If they charge the batteries slow enough they may not bother with checking for end of charge.

You should avoid using linear power regulators when using batteries. Linear power regulators regulate by appearing as a resistance between the load the battery. It is clear, using this paradigm, that such a regulator will convert power into wasted heat. Instead consider a switching power supply.

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    \$\begingroup\$ The term you are looking for is linear regulator. Not analog. All regulators are analog. In fact the whole world is analog, for practical purposes. Digital is just a useful abstraction. \$\endgroup\$ – mkeith Jul 24 '16 at 18:36
  • \$\begingroup\$ Good point, I'll change the answer from "analog" to "linear". \$\endgroup\$ – st2000 Jul 24 '16 at 18:40
  • \$\begingroup\$ Good point on the switching regulator. Can I just use one of those and treat the battery as a constant source, then when it stops working put the batteries in an external charger? or would I need an over discharge protection? \$\endgroup\$ – Owen Tourlamain Jul 24 '16 at 18:45
  • \$\begingroup\$ I would say that it is reasonable to connect a linear 3.3V regulator to a 3.7V lithium battery. If you run through the calculations, the efficiency is not that bad compared to a switcher. Likewise, if the current is exremely low (say 100uA, for example) it doesn't make sense to use a switcher. Generally switchers are not very efficient when Iout is 100uA. \$\endgroup\$ – mkeith Jul 24 '16 at 18:52
  • \$\begingroup\$ A buck switcher will reduce voltage and a boost switcher will raise voltage. Some switching power supplies have both. Discharge protection is usually needed for secondary lithium type batteries. There are probably COTS battery managment modules with what you want. \$\endgroup\$ – st2000 Jul 24 '16 at 18:55
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re: "Ideally I would like to have two batteries in parallel so I could get 4600mAh, but I know there can be issues charging and discharging in parallel, plus I have no idea if that charge controller will do that."

A simple technique to safely charge li-ion cells in parallel is to set the max V of your charging circuit to 4.15V. Charging will be slower than single cell charging and your cells won't achieve 100% charge; however, both cells will will eventually charge to a max of 4.15V (if they don't have any defects). If you use this charging method, I suggest you start with new cells from the same batch & lot (if possible) & always charge & discharge them together (i.e. don't pair them with other cells when charging or discharging the new cells). If your cells ever become out of balance (i.e. they have different voltages after charging) that means that the cells are not charging at the same rate. If you want to balance the cells, you can place 1 bar (or wire) across the + terminals & another bar (or wire) across the - terminals for a while & after some time (probably less than an hour) the cell voltages will become equal. A simpler, but more expensive remedy to unbalanced cells, is to replace the cells with new cells--if the V or charge difference is too great.

re: "Can I use anything advertised as charging Li-po on Li-ion?" No. You will need to use a charger that matches the charge requirements of your cells. There are many different li-ion chemistries that are available & not matching the charger to your chemistry can result is very bad results (including venting/explosion/fire/etc).

re: "One other option would be to put the two batteries in series and an LM7805 to bring the voltage down to 5V, would this be suitable?" 2 cells in series would have a max charge of 8.4V. The output of your LM7805 is 5V, so it wouldn't do you much good as the V is too low to fully charge the cells. You could placed the cells in parallel & use a current limiting resistor, to charge the cells, however, you will not have any overcharge protection circuitry to prevent overcharging the cells. If you included an overcharge circuit, then you'd only lack a balancing function & over-heating charge cut-off function--so, still not 100% safe, but doable if you charge in a metal box in a safe location & you monitor your charging cells.

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  • \$\begingroup\$ On the last point, I believe you misunderstood. The OP wants to use the 7805 not to charge the batteries, but to mitigate the dangers of using the batteries in parallel. The OP wants to charge them with a commercial charger, but use them in series with the 7805 to power the device, with no danger of one battery charging the other at too-high current. \$\endgroup\$ – piojo Feb 24 '18 at 4:27

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