Longterm Charging of Headway Cells With Dc to DC Converters

[Jimdear2]

We are using isolated DC to DC units to charge individual 3 cell stacks to a regulated 3.5 volts in a 50S/3P battery. This means that each cell stack will reach 3.5 volts and basically stop charging and just float. The three cell stacks have been matched as closely as possible for IR and voltage under a fixed load for a specified time, so there is no real variation of nominal cell voltage.

We are wondering what the effect of leaving the charging system powered up long term. Example, plug it in after the pull and unplug it next week end.

We have gotten conflicting comments about doing this. Some state it will cause the batteries to overheat swell up and blow out the rupture disc other say it will be OK and still others the weakest battery will be damaged.

Is there anyone with experience with this type of scenario that can offer comments.

We also welcome anyone's thoughts on this.

 

[texaspyro]

A123 specs a 3.45V float charge so you should be able to float charge the cells just fine (I'd check with Headway for their recommendation)

My testing shows there is virtually no difference in capacity from charging to 3.4V vs 3.6V See the first post in thread http://endless-sphere.com/forums/viewtopic.php?f=14&t=20698

 

[frodus]

When the battery voltage equals the DC-DC converter voltage setpoint, they will draw current until the battery equalizes, meaning, when the DC-DC converter stops PULLING the battery up to the setpoint and the battery finishes charge. At that point, they should not be drawing any current on the battery side of the DC-DC, you're only drawing power for the inefficiency of the DC-DC converter on the 48VDC side. It will be on, but the battery should not draw any current, or very little due to IR.

I've left headway batteries on overnight on a 3.7VDC output Vicor (48Vin, 3.7Vout and 75W) and measured NEAR 0 current (not calibrated equipment) the next day. This was after 24 hours. No heating, nothing. I did this on purpose. As long as you are NOT overcharging the battery, you're fine. This is exactly how a CC/CV charger works. It goes max current until you reach the voltage setpoint, then it keeps at that setpoint and current slowly decreases as the battery requests less and less. These DC-DC converters are NOT charge "pumps", they don't force current, they send current as requested by the battery.


And yeah, there is no difference in charging at 3.4 versus 3.6 as far as SOC, the difference is charge time. 3.4 will take longer because its not "pulling" the battery up to a higher voltage, so its drawing less amps, taking longer to charge. 3.4V is a lower C-rate charge, causing less heating, and slower charging, which is better for the battery, although either is fine as long as its below 3.65V (battery max charge spec).

 

[Jimdear2]

Texaspyro and Frodus,

Thank you both, this information is a big relief.

Texaspyro, following that thread about A123 batteries was very helpful.

We always try to check any negative information for accuracy, the thoughts that damage could occur if the chargers were left on for extended periods was really off putting.

One of the reasons we went the way we did with the chargers on board was we wanted to be able to float the battery. People warning us of bad consequences was nerve racking.

We are initially using a voltage of 3.500 volts for our finish charge specification, so far the voltage is settling out to just under 3.4 volts but that is with an ambient temp of 30 or so degrees. I'm sure once the weather warms that will rise to above 3.4 volts. We had already seen that charging to a voltage above 3.4+ volts was a waste of energy and time. We settled on 3.5 volts as a start because it seemed the best balance between heat and time, when we tried 3.6 volts, things just got too hot.

We are thinking of finding a way to heat the batteries to 100 degrees or so just before a pull. When we were testing we were amazed at how much better they performed when warmed up. What do you guys think about leaving the battery partially discharged and then fast charging at the track just before the pull as a method of heating the batteries. I wanted to try hot air but because we used the Headway blocks to build the pack there is insufficient air flow to be sure of even heating. In fact we are a bit worried about the center of the pack over heating when under a real load. Well I guess we will just have to find a happy medium.

Again thanks for your help.