Let's Make Robots!

Basic Battery Review

Alkaline (specially marked "rechargeable" only)Gradual charge loss, 1.5v/cell instead of 1.2v, low self-discharge rate, no toxic metals (throw them in the trash when done in the US)Fewer recharge cycles, diminishing charge capacity over battery life
Lithium Polymer (LiPo)Faster recharge, good "power to rate ratio" (?), several use cylces even in a day, relatively cheapThese batteries catch fire or blow up airplanes. Special disposal required.1.2v cells
Nickel Cadmium (NiCad)Cheap, formerly ubiquitousSpecial disposal required, harder to find because of NiMHs replacing them, memory effect, low capacity, 1.2v cells, relatively heavy
Nickel Metal Hydride(NiMH)Light weight, high capacity,no toxic metals (US)High self discharge rate of 25%/month, High capacity versions won't fully charge in all chargers, very heavy, 1.2v cells

That chart was all the original material I got  I totally pirated this from zbattery.com:

We frequently get asked the question, "How am I supposed to connect my battery if I want to double the capacity but not the voltage?", or similar questions. It can be confusing if you've never done it, but hopefully this'll make it simpler. Be sure to read the important notes at the bottom to protect yourself from damaging any equipment! 

Connecting in Series 
When connecting your batteries in Series you are doubling the voltage while maintaining the same capacity rating (amp hours). This might be used in a scooter, Power Wheels kids vehicle, or other applications. Just use a jumper wire between the negative of the first battery and the positive of the second battery. Run your negative wire off of the open connector from the first battery and your positive off of the open connector on your second battery. 

Connecting in Parallel 
When connecting in Parallel you are doubling the capacity (amp hours) of the battery while maintaining the voltage of one of the individual batteries. This would be used in applications such as laptop batteries, some scooters, some ups backups, etc. Use a jumper wire between the positives of both batteries and another jumper wire between the negatives of both batteries. Connect your positive and negative wires to the same battery to run to your application. 

Important notes: When connecting batteries in a pack there are some important things to keep in mind - - Find out the requirements of your application. For example: Don't double the capacity on your Power Wheels vehicle if you're not supposed to...you could burn up the engine. Follow the recommended guidelines for your application. - Don't use two different chemistries when connecting a pack. Usually the voltages will be different, but more importantly the charge rates will be different and the capacities may be different, thus resulting in a shortened life span. - Try to match capacities as much as possible. When connecting batteries in a pack you should try to match the capacities as much as possible to avoid discharging one battery quicker than another. A pack operates at a combined voltage so your one cell that discharges quicker will likely discharge deeper than it may be able to recover from. 

...and from rccartips.com: 

What to do to get the best performance from batteries

Secrets to making your batteries perform better and last longer...

  1. Discharge completely before charging.
  2. Use a peak detection charger.
  3. Charge at a consistent rate, usually 3.5 Amps.
  4. Use or cycle the battery (charge, full discharge) at least once a week. (Note: this does not apply to LIPO)
  5. Avoid using a battery more than once a day for NiCad and NiMH. For LIPOs you can use several times a day.
  6. Avoid charging a warm or hot battery. Let it cool first.

Tips: The higher the charge rate, the higher the voltage and performance, at the sacrifice of lifespan. I usually charge at a 3.5 Amp rate.

What not to do to your batteries

Avoid doing these at all costs...

  1. Overcharging. This will permanently destroy a battery pack.
  2. Do not let the battery get too hot. This may happen if you are over-geared, using the wrong motor, running the rc car in grass or uphill, or not enough air to cool the battery.
  3. Do not "short" the battery. This happens when you accidentally touch the positive (+) end with the negative (-) end.
  4. Do not throw batteries in the trash. Dispose of old batteries at a recycling facility.

(This next part I can't remember the source of-Max)

Volts, Amps and Watts

WATTS are the units of POWER. A hairdryer full-on might be 500 WATTS; on the low-power setting it might be 200 WATTS. The higher the POWER the bigger the charger.

VOLTAGE must be matched to the equipment in use (usually 5v for μCs, 4.5-6v or even 7.2 for servos, just about anything for motors...)

CURRENT indicates the flow of energy from the battery and is measured in AMPERES (or AMPS). Zero current and the battery is not discharging. The higher the current the faster the battery will discharge.

A battery is rated in AMPERE-HOURS (abbreviated Ah) and this is called the BATTERY CAPACITY. For example, a small boat might have a 12 volt 100Ah battery. This battery will provide 100 AMPERE-HOURS before needing to be re-charged. This may be taken from the battery as

1 AMP for 100 hours

2 AMPS for 50 hours

10 AMPS for 10 hours etc.

WATTS are VOLTAGE multiplied by CURRENT, so taking the above example with the 12 volt battery

1 AMP x 12 VOLTS = 12 WATTS for 100 hours

2 AMPS x 12 VOLTS = 24 WATTS for 50 hours

10 AMPS x 12 VOLTS = 120 WATTS for 10 hours

Re-charging a battery follows the same principle. The requirement is usually to re-charge the battery over-night - say in 10 hours.

Because a battery is not totally efficient at converting electrical energy into chemical energyand vice-versa, re-charging a 100Ah battery requires about 120Ah to be put back into it, andthis can be achieved by either

120 Amp-hours / 10 hours = 12 Amps for 10 hours

120 Amp-hours / 15 hours = 8 Amps for 15 hours

120 Amp-hours / 24 hours = 5 Amps for 24 hours etc.


therefore if, say, the lights add up to 36 WATTS and the battery VOLTAGE is 12 VOLTS then the CURRENT taken from the battery will be

36 WATTS / 12 VOLTS = 3 AMPS

If these lights are on whilst the battery is being charged, then the battery charger must also provide an extra 3 AMPS to power them.

Hope that's useful for people using more than 4 AA cells!


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uhm some points on that table:

-alkaline is NOT rechargable, they are single use primary cells.

-lipo cells are not strictly 1.2 volts.
in fact most cells i have seen are 3.6 or 3.7 (rcr123, 14500, 18650 etc)

also on the exploding issue:

lithium cells seem to 'vent' mostly when they are used in series.
the trouble comes when they are unevenly discharged and one
cell either reverse charges or overcharges another, which then
will vent.

venting = the production of a gas that will ignite upon contact with oxygen
producing a jet flame.
actual explosions only happen when they are sealed tight inside their case.
it is the case that explodes due to the build up of pressure, not the cell.

The very first rechargeables on the market were alkaline. They're hard to find now if availabe at all, but they did let you recharge them to a crappy extent. You aren't meant to recharge a standard alkaline cell, but specially marked ones would take a charge better than regular ones. I only inculded them in case they were still available in some remote corner of the world. (As I've been reminded time and again, users of LMR are not all in the USA, which is why I noted our disposal requirements. Chances are they won't be at the top of the strictness pile, but you know if the US has an environmental restriction no one is skeptical about the need for it!) I will note the difference though. As to the exploding planes, I'd hoped it was obvious I was being facetious. Airlines have some restrictions about those batteries though because of an incident where a UPS plane full of them caught fire over Dubai I think it was. (I had to look that up whan my boss bought her new Android tab and it had sticker on the box that said not to take it on airplanes.) CtC has said he won't stock them at RocketBrand for fear of starting fires. They are the most dangerous type of battery available in most regards though. I've never disassembled a LiPo cell. I assume a 3.6v cell is really 3 smaller 1.2v cells, the way some 9v batteries are actually 6AAAA cells.

It caught my attention when you said, "They are the most dangerous type of battery available in most regards though."

That might depend on your definition of dangerous. I thought immediately about nuclear batteries as potentially the most dangerous. No, they will not explode like a nuclear bomb, but the radiation itself escaping from the battery could be dangerous to living tissue.

Several places on the internet may be found describing different types of these. Here is one of them. http://peswiki.com/index.php/Radioactive_battery

ile some have been used in orbiting satellites, there are some that are proported to be safe enough for use in pacemakers, but I have my doubts about that.   :-)

the ones i have in mind are all single cell 3.7v nominal:

the ones you find in cellphones and such tend to be single cell ones at 3.6v
though the bigger ones in laptops might be battery packs.

incidentally the problem with planes lies with the fact that they tend to carry fire extinguishers
that are insufficient to put out a lipo battery fire.

the ones i have in mind are all single cell 3.7v nominal:

the ones you find in cellphones and such tend to be single cell ones at 3.6v
though the bigger ones in laptops might be battery packs.

You are right about the LiPo terminal voltages, but I differ on alkaline batteries. They will recharge but do so poorly. You might get 5 or 6 charges with each one taking in less and less charge.

Nice post Max.

Dan M is an engineer and has added the following insights at this node:

Battery packs that are different could produce eddy currents that are better avoided.

You could use a relay, held energised by the primary battery and when it drops out would switch to the backup battery.*

I might be more prone to wire both battery packs "almost" in paralell. By that I mean place a Schottky diode in series with each. Those diodes have a low forward voltage drop, typically about 0.2 volts across them. (0.15 to 0.3 range ). Anyway, whichever battery is the highest, it will supply current to the circuitry, and nothing will flow either to or from the lower voltage battery.


*Doing this would avoid one of the problems I mentioned, namely that there might not be enough power to trigger the relay to the second pack if you relied on the first pack to do it.  However, it does also use up some of the battery's charge holding the relay energized.

Quite so. Because a relay would use up current holding it energised, is why I opted to simply paralell the batteries in my Schrödinger robot, but with schottky 4-layer diodes in series with each set. In addition, if you were intent on switching the batteries as in the relay example,  would suggest making (or buying) a "solid state relay". You can use transistors, but using FETs for the switching would use even less energy than regular transistors and much less than holding a relay coil energised continuously.