combining two 3.7v drone batteries to get a 7.4v battery

Thread Starter

flash01694

Joined Mar 7, 2018
26
hello guys, for a project of mine I require a battery which is more than 5v but is really compact. We are talking 20 x 15 mm on the two most narrow sides, and not more than 70mm on the longer one. Unfortunately, there are no batteries like that on the market as far as I saw.

my workaround would be to get two of these 3.7v batteries and put them in series

http://keenstone.com/6-pcs-of-keens...dcopter-compatible-with-walkera-super-cp.html

I'm not sure however if it's going to work. On the website it states there is a bms with overcharge protection, will that prevent the battery from working if it detects 7.4v?

also, if I do use this 7.4 battery how do I charge it up? with a 9v charger?

thank you guys!
 

dendad

Joined Feb 20, 2016
4,451
You should be able to run them in series to get the 7.4V I expect.
To charge the batteries you disconnect them and plug then each into a separate connector of the charger. It may require a 9V to 5V regulator for the charger so it can run off your 9V supply. You will need to check the charger info.
3Amp Buck.jpg
These 3A regulators are available on Ebay for a couple of dollars. Some are preset to output volts and others have a trim pot.

Make sure you use a proper LiPo cell charger unless you want a fire/explosion!
 

Thread Starter

flash01694

Joined Mar 7, 2018
26
You should be able to run them in series to get the 7.4V I expect.
To charge the batteries you disconnect them and plug then each into a separate connector of the charger. It may require a 9V to 5V regulator for the charger so it can run off your 9V supply. You will need to check the charger info.
View attachment 152156
These 3A regulators are available on Ebay for a couple of dollars. Some are preset to output volts and others have a trim pot.

Make sure you use a proper LiPo cell charger unless you want a fire/explosion!
I already have the right lipo charger for the 3.7 v and you are bringing up a good point here, I might as well do the series wiring on the Arduino end so I can detach each of them on their own. thanks!
 

RichardO

Joined May 4, 2013
2,270
Rather than adding a second battery you might be better off using a step up switching power supply to get the higher voltage.

Tell us more about what the 7.4 volts is going to be used for.
 

ian field

Joined Oct 27, 2012
6,536
hello guys, for a project of mine I require a battery which is more than 5v but is really compact. We are talking 20 x 15 mm on the two most narrow sides, and not more than 70mm on the longer one. Unfortunately, there are no batteries like that on the market as far as I saw.

my workaround would be to get two of these 3.7v batteries and put them in series

http://keenstone.com/6-pcs-of-keens...dcopter-compatible-with-walkera-super-cp.html

I'm not sure however if it's going to work. On the website it states there is a bms with overcharge protection, will that prevent the battery from working if it detects 7.4v?

also, if I do use this 7.4 battery how do I charge it up? with a 9v charger?

thank you guys!
Cells in series happens all the time - but only with identical cells.

You need some kind of charge balancing circuit. You can buy off the shelf modules, you can use 2 identical floating output single cell chargers or you can take the cells out and charge them in parallel. It gives the best balancing, but you need to equalise the cells by connecting via a resistor before dumping them on each other or destructive current could be fed from one to the other.
 
You need some kind of charge balancing circuit. You can buy off the shelf modules, you can use 2 identical floating output single cell chargers or you can take the cells out and charge them in parallel. It gives the best balancing, but you need to equalise the cells by connecting via a resistor before dumping them on each other or destructive current could be fed from one to the other.
Please expand this with more detail. Suppose I have two cells which have slightly different voltages--I calculate the resistor using the current I want and the difference between the cells and let them sit till they have the same voltage? Say a difference of 0.1 volt and I want 100 mA I'd use a 1 ohm resistor? How long would I wait? Roughly. Thanks
 

ian field

Joined Oct 27, 2012
6,536
Please expand this with more detail. Suppose I have two cells which have slightly different voltages--I calculate the resistor using the current I want and the difference between the cells and let them sit till they have the same voltage? Say a difference of 0.1 volt and I want 100 mA I'd use a 1 ohm resistor? How long would I wait? Roughly. Thanks
For scale/perspective - many e-cigarettes using an 18650 cell are less than 1R.

You're in the right ball park and less than half hour is probably enough to blunt the imbalance.
 

MisterBill2

Joined Jan 23, 2018
18,176
If you are connecting the two battery packs in series there is no need to equalize the voltages, no need at all. Parallel operation, possibly although I routinely connect two 120 amp-hour gel-cells in parallel for occasional high current service and they still last many years. The particular high current application is powering a bank of 24 thousand watt floodlights for video recordings of automotive crash testing. So the time of the high power operation is usually about 30 seconds for the slow crashes. That is exciting to watch, by the way.
 

ian field

Joined Oct 27, 2012
6,536
If you are connecting the two battery packs in series there is no need to equalize the voltages, no need at all. Parallel operation, possibly although I routinely connect two 120 amp-hour gel-cells in parallel for occasional high current service and they still last many years. The particular high current application is powering a bank of 24 thousand watt floodlights for video recordings of automotive crash testing. So the time of the high power operation is usually about 30 seconds for the slow crashes. That is exciting to watch, by the way.
The discussion is about lithium - not lead acid.

Charging series lithium cells without charge balancing is a seriously bad idea.
 

Thread Starter

flash01694

Joined Mar 7, 2018
26
Cells in series happens all the time - but only with identical cells.

You need some kind of charge balancing circuit. You can buy off the shelf modules, you can use 2 identical floating output single cell chargers or you can take the cells out and charge them in parallel. It gives the best balancing, but you need to equalise the cells by connecting via a resistor before dumping them on each other or destructive current could be fed from one to the other.
okay thanks!

so basically I would want to connect the positive side of one of the two packs to the negative side of the other with a resistor and vice versa?

I am actually using the same exact charger to charge both of the batteries at the same time, is balancing still required?
 

MisterBill2

Joined Jan 23, 2018
18,176
I had been thinking that the simplest approach would be charging one cell at a time with the charger. It would take longer but there would be no need to do anything else except connect the charger. The time would be the trade-off item. Post #2 suggests exactly this, and then goes on with other concepts as well. The question becomes mostly "how much POWER does your application require?" There are step up converters that will give about any boost that you need available. Depending on your exact voltage requirement, you may also be able to utilize an isolated 3.3 regulated supply and put that in series with the 3.7 volt battery and get 6.7 volts.and that would use only one cell, so no equalizing needed, and in addition it could be really small. My suggestion is to see what Digikey offers, because they have a great catalog, and then find it on Amazon to purchase it.
 

ian field

Joined Oct 27, 2012
6,536
okay thanks!

so basically I would want to connect the positive side of one of the two packs to the negative side of the other with a resistor and vice versa?

I am actually using the same exact charger to charge both of the batteries at the same time, is balancing still required?
Some cells have the charge control circuit embedded - yours probably don't.

The most common scheme is a regular ish wall wart and the charge control is more local to the cell.

The maximum safe terminal voltage is *VERY* specific, it matters less how you comply with that as long as you do.

You could use a TL431 shunt regulator for each cell, but you have to have shottky barrier feed diodes so a shunt failure cant dump a fully charged cell. not difficult on 2 cells - but gets pretty tricky on 3 or more.
 

ian field

Joined Oct 27, 2012
6,536
Charging one cell at a time with the single cell charger is what I had been suggesting, since that was available. Then there is no need for charge balancing.
I'd do both at once in parallel - you can avoid all the mucking about with current equalising by using a very low resistance "Y" splitter.
 
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