Have an idea, but not sure of implementation

[Roy Von Rogers]

Hi all.

Having optained two A123 developer packs some years back, and wanting to use them in some ways, brought me to this site, and after reading many of the posts, got the E-bike bug. Also having bought on of Gary's boards and planing on building it at a later date, had an idea, but could use some advice from those in here who have a little more knowledge then I.

Since A123 cells are 3.3v and looking at how to charge, it struck me that since I have build a lot of computers, the power supply on those all have 3.3v with some hefty amperage. So, if one was to get a cascade chip, like a led chaser chip, and slow it way down where it would say hold for 5 seconds before it moved, could one use those to trigger a switch system to go from one 3.3v parallel pack to the next. ? I'm a big believer in not stuffing and would rather buy extra batteries and keep them at 80% level, to get longjevity. My first simple implementation idea was to use relays, which means a lot of relays since one would need two for each cell(s), one negative one positive. The signal from chaser chip would trigger lets say the negative relay and that relay would trigger the positive one. This would also be a safety in case a relay got stuck.

Now one could do it with an fet or transistor, but one would loose some voltage trough the device, so one would have to up the voltage on the supply, to negate the drop. And getting a single adustable 3 volt high amp power suply would also be a way to charge this way. This way one would charge the cells to the optimum 80% charge, and be sure all cells have the same voltage, for its getting charged from one source.

Need some input here to see if this idea is workable.

Roy

 

[fechter]

I would call this a sequential charger.

The concept could be made to work, but there are some challenges:

You really need 3.65 - 3.7v to charge LiFePO4 cells. 3.3v is not quite enough. Most computer power supplies are adjustable over a small range, but this varies by particular model. Likewise, you might be able to turn down the voltge on a 5v supply.

The switching between cells could happen at a higher speed. The switching speed is probably not very critical.

For each cell, you need a pair of switches. These would most likely be FETs, but a mechanical relay could work at very low speeds. The switches need to handle the full charging current and the pack voltage. For a 16s pack, this would mean 32 FETs. The real challenge with using FET switches is getting the gate drive voltage. At low speeds, you might get away with a fairly crude setup, like a zener diode and a resistor.
The switches need to handle the full charging current and the pack voltage.

 

[Roy Von Rogers]

Looking at my A123 charge sheet the recommended float voltage is 3.45v. So I'm assuming one could charge to that point and not having to worry about reducing the amperage, and not charging to 3.6v will extend the battery life by a good amount. I know many like to get all they can out of a cell, but going to the max at both ends is similar to an ICE, just because you have 200 hp available doesnt mean you should use the constant hp, without shorting the life of the engine.

To be honest, I like the squencial charging, for if you use a power supply set up with the proper charge rate, all the cells will be exactly alike, if its a good pack. The main reason for using relays was the no loss (exept line) from the relay contacts, and the reason for 5 seconds or longer was the keep the contact arcing down. If one looks at the max wattage on the charger and the time it would takt to fully charge the pack, one would just divide that time by the pack wattage, and then figure out how long to keep each relay active to give all cells time to fully charge.

The max recommended charge rate on a A123 cell is 10 amps, so if you have 12 cells in parallel the charge rate would be close to 30 amps, which is about as close to what one would use considering whats available out of a 120v wall plug, depending what type of power supply one uses.

Roy