Li-ion battery charging and usage

Question

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.

Answer 1

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.

Answer 2

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.