Charging A123 M1 Lithium battery packs

As of the middle of 2008, there are a number of chargers and balancers suitable for 10 cell A123 packs.

Automatic stand-alone balancers

Note that most balancers work by discharging the high cells and only use a balance current of between 50 and 150 mAmps, so can take several hours to correct a badly balanced pack.

Combined chargers and balancers

There are a large number of Lithium chargers/balancers for 6 cells or less and most are A123 compatible these days.

To avoid the hassle of connecting the balance leads when the packs are in the robot, we need to have the cells in a pack well balanced beforehand, so that no single cell will be over charged when we use a simple pack charger. At high charge currents (> 2 amps), there is risk of cell damage if the cell voltage goes over 3.7 Volts.
Once balanced, the M1 packs would seem to keep their balance quite well and not require frequent balancing.
So we can use a simple non-balancing fast charger at competitions (see below) and then when we get home we can balance them.

Note that you should not use a standard LiPo charger with these cells. A123 M1 cells have a significantly lower full-charge cut-off voltage than LiPos.
If you have a NiCad charger capable of charging a 36V battery packs e.g. Astroflight 112, then you can use this with a Dapter.
The latest version of the Astroflight 109 Lithium charger is possibly compatible with M1 cells - you should check that it is.
The latest versions of the Schulze Chamaleon isl 6 chargers can also charge M1 cells directly. You may be able to get a ROM upgrade to the latest version.
Note that the above chargers will not balance the cells. They are effectively just a step-up power supply (to step up the voltage from the input 12 V), a display and a voltage cut-off. Also, if you are charging four 36V packs at 5 amps, this is over 700 Watts. Not many of the PSUs that people currently use can supply that kind of power.
For charging the packs at a competition, the current plan is to use some 48 Volt power supplies and a simple voltage cut-off - the SLK Electronics Dapter. see the Fast charging section below. The Dapter does both normal LiPos and the A123 M1 cells. If you do not need to use them with LiPos, he does a version that is set for a fixed number of M1 cells which is easier to use - you don't have to tell it that it is charging M1 cells or confirm the cell count.
Found a couple of 10 Amp 48 Volt switched-mode power supplies on ebay for around £50 each. These can be used to charge from 1 to 3 packs, depending on the current you want to charge at. We're planning to charge two packs off each charger at around 5 amps per pack. The Dapter can handle up to 8 amps.
There are 48V PSUs you can get that have a smooth current limit built in e.g. the Meanwell USP-225-48 (4.7 Amps, around £70 each).
With the PSUs we are using, we will need some form of external current limit. A 55 Watt car headlamp bulb for each pack should do nicely.

Manual balancing

You will probably want to make a simple 'break-out box' so you can manually balance the packs when you first make them up. When they are first made up, the packs can be substantially out of balance, especially if you have replaced a cell.

You can get them roughly in balance when the cells are partly charged, but to get a good balance, this has to be done when the cells are fully charged. We only need the cells to be balanced at full charge, so that all the cells peak together and no cell is over-charged.

The cells would only have to have very slightly different capacity for a pack that is perfectly balanced at 50% capacity to be significantly out at full charge.
It is also much easier to get a good balance when they are fully charged, beacuse a small difference in charge results in a relatively large change in voltage.

To dischange a cell, I use three one Ohm resistors in series. I use only one amp discharge to minimise the voltage drop in the balance wires, which confuses matters.
To charge a cell I use a 0.1 Ohm resistor from my variable power supply, set to around 4 Volts. Vary the voltage to alter the charge current between 1 and 5 amps, depending or requirements.

Packs should only be balanced when they are fully charged. The voltage of the cells when they are say 50% charged is not necessarily indicative of their state of charge.

The only purpose of balancing the cells in a pack, for our application, is to ensure that when we are fast-charging with a simple (whole pack) voltage cut-off, that we do no over charge any individual cells. They may not run out of charge at the same time at the end of a full discharge (due to different cell capacities), but we should never be doing full discharges. The only thing we need is for them all to be at more or less the same voltage at the end of charge.

Fast charging

To charge the battery packs at a competition, you need three things:

Three alternative ways to charge four Lithium packs at competitions. None of these setups will balance the cells - just simple voltage cut-off charging.

1 customised 48 Volt 500 Watt PSUs
  • 2 x 48 Volt 500 Watt power supplies - around £50 each on
  • 4 x 12 Volt 55 Watt H3 car headlamp bulbs (mounted in heatsinks)
  • 4 x Dapter voltage cut-offs
This is a much cheaper solution (if you have to buy all the equipment) than the equipment that you would need to charge a similar number of NiCads, which is similar to the solutions below.
2 1000 Watt 13.8 Volt PSUAstroflight 112 NiCad chargers
  • 1 x 13.8 Volt 1000 Watt power supply - around £140 on
  • 4 x high-power (36V+) NiCad chargers e.g. Astroflight 112
  • 4 x Dapter voltage cut-offs
3 1000 Watt 13.8 Volt PSUAstroflight 109 Lithium charger