Charging A123 M1 Lithium battery packs
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, so that no single cell will be over charged.
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. 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.
Balance charger
For a balancing charger, we have been investigating the DeWalt DC9000 36V one hour charger,
using an adapted DeWalt battery case to connect it to our packs.
Converted the empty case from a DeWalt DC9360 battery to allow the connection
of a built-up pack to the DeWalt charger. Do not connect the first and last balance wires. The DeWalt charger uses the main
connectors for those.
NOTE: Never connect or disconnect any of the leads between the battery case and
the pack while the battery case is plugged into the charger.
This will damage the electronics module inside the case, as I have found out several times!
Some graphs of the cell voltages during charging with the 36V DeWalt charger. In the first two, I had deliberately added some charge to one or two of the cells before the charge started, to see how it handled imbalanced cells.
The charge appears to be in three phases:
- Main charge at around 3 Amps, pulsed. First and then second lights flashing.
- Peak at 1.6 Amps, pulsed. Third light flashing.
- Balance at 150 mAmps, pulsed. All lights on solid.
Currently I'm not sure if these chargers are suitable for our purposes. They have a very effective, if slow, balancing system, but they do not seem to carry on balancing for long enough.
Quite often they do not even bother balancing, unless the cells are quite far out. While they ensure that no cell is overcharged, they may not do a good enough job of balancing the cells to enable the pack to be charged with a simple pack charger safely.
The pulsing is presumably just so that it can get accurate voltage readings from the cells, without being affected by voltage drop across the balance wires i.e. it only measures the cell voltage when the current is switched off.
The balance wires within the DeWalt pack are made of spring steel wire, and therefore have rather high electrical resistance - up to 300 mOhms each.
Note that the balancing only occurs AFTER all three lights on the charger come on solid, so you should leave the battery connected for at least 20 mins after the three lights come on.
It is not clear how it decides to stop balancing, but it rarely seems to get all cells perfectly balanced.
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.
Automatic stand-alone balancers for 10 cells or more:
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 10 cell charger and balancers:
There are loads of Lithium chargers/balancers for 6 cells or less and most are A123 compatible these days.
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.
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:
- A power supply capable of supplying at least 40 Volts
- Some means of limiting the maximum current
- A voltage cut-off to switch off the charge when the cells are full - 3.6 to 3.7
volts per cells i.e. 36 to 37 Volts for a 10 cell pack.
Three alternative ways to charge four Lithium packs at competitions. None of these setups will balance the cells - just simple voltage cut-off charging.
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- 2 x 48 Volt 500 Watt power supplies - around £50 each on
eBay.co.uk
- 4 x 12 Volt 55 Watt H3 car headlamp bulbs (mounted in heatsinks)
- 4 x Dapter voltage cut-offs
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- 1 x 13.8 Volt 1000 Watt power supply - around £140 on eBay.co.uk
- 4 x high-power (36V+) NiCad chargers e.g. Astroflight 112
- 4 x Dapter voltage cut-offs
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