# How to make a balanced battery pack?

I am making a robot for my workplace right now. I started with a 20servo kit. It came with a really terrible 10.6volt, 800mAh NiMh battery pack though.

I would like to make a lithium pack for this since 3 lithium cells would be perfect. I use LiPo for my personal model airplanes and I know they are "balanced" and they use special chargers that charge each cell independently.

Since this is for my workplace I do not want to use LiPo because they are so volatile. I was thinking of making my battery pack out of 3x RCR123's. I would like to make the pack so it can be charged with a balancing charger.

Can someone please tell me how to do this? Also, can I use a LiPo charger for Li-Ion or do I need to make my own charger?

Thank you.

• IIRC Li-Ion is not compatible with Li-Poly from a charging perspective – vicatcu Jun 28 '12 at 22:53
• Thank you for that. But can I make a balancing charger for a custom LiIon pack? – Sponge Bob Jun 28 '12 at 22:54
• @vicatcu Are you sure? From everything I've read, they charge exactly the same and in many charger IC datasheets same process is described when charging Li-Ion and Li-Po. – AndrejaKo Jun 29 '12 at 5:35
• @Keegan McCarthy There are in general two types of balancing charger and both require that there is a way to access each cell individually. One type charges cells in series and every few seconds checks the voltages. If one cell has higher voltage than other cells, it will be discharged until its voltage meets the voltage of other cells. The other type of charger charges each cell individually. This should decrease chances of having unbalanced cell. It uses same algorithm to check if a cell is balanced and balance it if needed. – AndrejaKo Jun 29 '12 at 7:04
• The first type seems to be a bit older and sometimes is used with dedicated balancers which will monitor the voltage independent of charging of the battery. The second type seems to be a bit newer and the balancer is integrated into the charger. – AndrejaKo Jun 29 '12 at 7:06

Since this is for my workplace I do not want to use LiPo because they are so volatile.

Lithium Ion cells are MORE susceptible to "vent with flame" / "magic smoke" events than LiPo (Lithium Polymer), not less.

If you want a much safer Lithium based chemistry cell then use LiFePO4.
(LiFePO4 = Lithium Ferro Phosphate).
You can drive a stake though their hearts (metal or not, at cross roads and at midnight optional) and they will not produce high energy discharges. They will (probably) not work very well thereafter

I would like to make a lithium pack for this since 3 lithium cells would be perfect. ... Can someone please tell me how to do this? Also, can I use a LiPo charger for Li-Ion or do I need to make my own charger?

LiPo and LiIon have similar charging needs.
LiPo usually allows larger charge rates but is essentially the same chemistry.
A charger must "respect" the maximum battery voltage and maximum charge current - these are usually selectable. The per cell termination voltage for LiIon or LiPo is usually 4.2V at 25 degrees C. Using a slightly lower voltage than this reduces capacity but greatly enhances cell cycle life.

A LiPo or Lithium Ion cell provides from 3.0 to 4.2 V/cell (discharged to fully charged) and cells are rated at 3.6V or 3.7V nominal A LiFePO4 cells provides about 2.8 to 3.6V cell and are rated at 3.2 or 3.3 V nominal. (Lower is possible but there is little capacity in the "tail" and using it reduces cycle life). A NimH cell produces about 1.0 to 1.3 V/cell and is rated at 1.2 V nominal

So your 10.6V NimH pack = 10.6/1.2 = 9 cells

Across charge range:

• 9 x NimH = 9V - 11.7V , 10.8V mean
• 3 x LiIon = 9V - 12.6V , 10.8V mean
• 3 x LiFePO4 = 8.4V - 10.8V, 9.6V mean
• Lead acid = 12+ V mean

You can stop discharging the LiFePo4 cells at somewhat higher than 2.8V so that the range is say 9V - 10.8V

You will need to determine how low a voltage the servos will run on happily but the LiFePO4 cells will probably be acceptable and are far better for safety. That said, properly run LiPo cause reasonably few problems.
Using 4 x LiFePO4 cells could be considered.

You may wish to consider the use of a 12V lead acid battery. These are cheap, a wide range of capacities are available and chargers are readily available.

You could use a boost converter from a lower voltage or a buck converter from a higher voltage to get good constant voltage operation.

### Balanced charging

LiPo cells can be charged using readily available "balancing chargers" or charged separately in parallel or you can make your won charger. A range of balancing charger ICs are available from eg Digikey.
A simple method to ensure balancing is to place a clamp regulator across each cell which shunts current when voltage reaches the constant voltage charge point. There are various ways of doing this but one such is shown below. This is from here. Good notes on operation and adjustment are shown on that page.
The IC monitors battery voltage and when a trigger voltage is reached the MOSFET is switched on to shunt current. The IC contains an internal voltage reference and provides programmable hysteresis for the clamp voltage. Use of one of these circuits across each cell allows use of a non balancing charger up to the current capacity of the clamp.

Black belt holders only In fact, although this would not be recommended, you could probably use a simple 12V lead acid charger with a current output no higher than the capability of the clamp circuits to charge 3 x LiIon or LiPo cells ion series. This would not provide some of the finer points of LiIon / LiPo charging but should work.

MAX921 datasheet. These are far too expensive at $4.60/1 Digikey. Same result can be had with a TL431 and maybe a few transistors. A workable circuit can be arrived at if these is enough interest. • Thanks for the awesome answer! I did indeed consider LiFePo cells (if you look at the question's revision history you will see that) I really do love LiFePo's I even made my Jeep's battery with them. But the voltage of the cells just isn't compatible with my robot. The servo's don't function well with less than 10.0volt. They are controlled by serial commands, not PWM, and from what I've observed something with the communication starts to go bad at a low voltage. However, I did not realize that LiIon are more dangerous than LiPo. I have exploded many LiPo cells, but never a LiIon. – Sponge Bob Jun 29 '12 at 15:55 • Maybe I'm just using sh*tty LiPo chargers. I'll take a look to see if I can find any breadboard-mountable balancing ICs and that might solve my exploding LiPo cell problem (and save me a ton of money... at 9-13 dollars for a good 1.3Ah cell...) – Sponge Bob Jun 29 '12 at 15:58 • ebay.com/itm/… ----- there's two of them for 8.4$ – Sponge Bob Jun 29 '12 at 15:59
• @KeeganMcCarthy - 4 x LiFePO4 + a voltage regulator would work well. Cycle lifetime should be 2 x to 4 x that of LiIon or LiPo. – Russell McMahon Jun 29 '12 at 17:12