Max safe MottCell charging voltage

Every lithium chemistry is slightly different and so it's hard to say without doing a complete analysis of the battery but I would say that 3.5v is a safe charging cutoff yes. You may be able to go a bit higher depending on how quickly the voltage rises...

The 3.2v rated voltage is the nominal voltage (or in other words, the average voltage over the full range of the battery), not the maximum voltage. Keep in mind that the voltage will drop once the charger is cut off.

Andrew

 

without having done detail analysis, but having watched how quickly the Mottcell voltage climb once it hit 3.5v; I would say that it is definitely in the knee region of the curve. At that point, the voltage very quickly rise to 3.75v and it is "probably" 95% charged.

Pb

 

The real question is when do we risk damage to the cells. Jack has indicated that going above 3.8v for 2 munites is OK. The key here is to limit the time above the voltage where the cell hits the knee in the curve.

If you have DB8s and no other monitoring then you would be best to shut down at 3.5v. I run CellLogs with loud alarms. I never miss an alarm and can get the charger shut down quickly. So I set the alarms at 3.96v based on what I have seen in charging cycles for the Enginer MottCells.



These are 4 charge cycles on my 4k system. The time for each full chart is 16 minutes. So you see that I am allowing high cell voltages for a very short time before the charger shuts down and for a total of no more than 10 minutes including the time for the cell to drop back down. This is safe.

If you were to do charging while checking the DB8s every 10 minutes you could see a peak of 4.1v to 4.2v and a total time above 3.8v of 30 minutes or so. This time at high voltage is what has damaged cells.

Thanks,
Dan

 

Dan, do you find that it is always the same cells that repeatedly reach the voltage knee ? In general, it is usually my cell #6, but i've seen also cell 7 & 8, and 5 sometimes. What gives ? are the cells still being broken in ? i can understand cell 7 & 8, as these were not drained prior to the mod to the cellLog.

thanks for any insight,
Pb

 

Yes, Which ever cell get full first goes high. Theoretically the balancer should correct this. However. The balancer tends to balance "at rest" because that is where the cells are the majority of the time.

You can manually correct this. Go to Radioshack and get two 1ohm10w resistors. Put them in series to get 2ohms. After charging put the 2ohm resistor across the highest cell. Be careful the resistor will get very hot. 2ohms will draw 1.75a. Do this for about 35 minutes. That would be about 1ah. Then drive the car and charge and check for the new high cell and do it again. In about a week you will be better balanced at the top. Note that the DB8s can cause the same cell to keep going high even when you make adjustments. You may get to the point where you want to unplug the DB8s. If you have CellLogs with alarms then go ahead and unplug the DB8s because the CellLog alarms will protect you from having any cell going high or low.

By the way, be sure that the same cell is not also going low at discharge. If that is happening then you have a capacity problem rather than a balance problem.

Thanks,
Dan

 

By "whichever cell get full first", are u saying that it is always the same cell going full first or different cell going full for different charge ?

I don't have the DB8 connected, just cellLog. I think I have capacity problem, even with buddy paired configuration. That is the reason I want to do the high/low cell capacity matching as you did.
Pb

 

Email Me your data from the Celllog in xls form. Include both a full charge and a full discharge. Then we can see what you have.

Thanks,
Dan

 

Hey Dan and others. Would this be a suitable charger for charging individual 3.2V mottcell enginer kit cells or 3.2V buddy pairs? I want my spares charged up before I put them in my kit.

http://cgi.ebay.ca/Charger-single-LiFePO4-TS-SE-cell-3-2V-3-65V-10A-/120533675315?pt=LH_DefaultDomain_0&hash=item1c105ded33

 

Hi Fibb,
It is vary hard to tell from information in the spec sheet how this chagrer turns off. From what it does say, I would guess that it would be OK. However I would still watch the cell voltage very closely for overcharging.

I use various different chargers. I always monitor the current and I always monitor the cell voltage with cellLogs and alarms so that I never overcharge a cell.

Thanks,
Dan

 

Thanks Dan,

Does this statement make any sense?

The Charger will cut off automatically if battery is full (1/10C)

Any other chargers I should buy instead?

Can I charge a buddy pair with this thing?

Do I need to remove the cell(s) I want to charge from the rest of the kit first?

I stopped doing that when I was charging 4 cells totaling 12V with my smart LA charger. Seemed to work fine leaving them in there connected to the kit. But that was before I had a buddy cell system. Mostly I will want to charge cells individually outside of the kit.

 

I've thought about buying this charger but have always been perturbed by the high shipping cost ($22.99). The implication of the 1/10C statement is that the charging will terminate once the constant current phase drops to 1 amp (1/10 of 10 amps). While it is handy to have a single cell charger, using one with a 10 amp current output is overkill if all you need is some fine-tuning. I love my 2 amp Tenergy charger and highly recommend it for bottom balancing, but those have been out of stock for several months. Also, it should not be necessary to charge cells outside of the kit on a regular basis.

 

Yes the shipping cost is quite high.

so is that safe enough if I want to top up a mottcell that might be 85% full anyway?

Yes I'm just doing fine tuning unless the cells I've had on the shelf have leaked out most of their juice. Not sure where they are SOC wise.

I just wanted to charge up a cell before I put it into service for the first time into a fully charged kit.

Out of my 16 pairs in the buddy cell arrangement, 1 pair is persistently lower in voltage than the majority and 1 pair is persistently higher than the majority.

I thought I would replace both pairs and hopefully expand the usable kWh in the kit.

My buddy cell smart balancer prevents overcharging of any buddy pair so the one high voltage pair stops the rest of the cells from getting a full charge.

At the other end, the converter quits perhaps a little earlier than it should because of the one buddy pair that is persistently lower in voltage.

Any advice on how to tackle this problem?

I also thought it would be handy to have a 3.2 V charger to condition the batteries that are being kept on the shelf if they do discharge over time.

 

I would suggest going to the enginer forum and requesting user status. There is a lot of information in the user forum that is not available to the general public.

Thanks,
Dan

 

Sorry can't find it. Got a link?

 

There is a link on the Enginer web site, but here is another link...
http://www.enginer.us/forum/index.php

Thanks,
Dan

 

I run my charger on a timer that turns it off for a half hour, then back on for a half hour, several times overnight. The theory is that the balancers will drain the top cell(s) when the charger is off, then all cells, including the low cells, will get charged more when it comes back on. So far, it seems to be working well. My system was installed in Dec 2009 by 3ProngPower using an Australian BMS with an over/under-voltage cut-off relay.