Optimate Lithium TM-291 12v charger / balancer

[PNjunction]

Some of you may know my own distaste for external balancing boards on simple 4S lifepo4 batteries for a variety of reasons.

I have to say that in front of my own eyes, the Tecmate-Optimate TM-291 lithium charger actually does the job without using individual charging leads. It has nearly perfectly balanced my own 20 and 40ah GBS 4S lifepo4 batteries. They say it can handle up to 100ah, but I don't have that large of a battery yet.

The reason it does this is that it does NOT apply just a CV at the end of charge, but a variable oscillation from 14.1 to 14.3. Each cell reacts differently during the minsiscule lifepo4 absorb curve, and if it didn't oscillate, current would just stop prematurely on an unbalanced pack. By oscillating it, during the time it takes to go back up to 14.3v at each oscillating cycle, the undercharged cells slowly catch up.

Ordinarily under a standard CC/CV routine, this would not happen as you'd expect and no balancing would occur.

I proved this to myself by purposely severely unbalancing my pack, and having a go at manual balancing via an RV incandescent bulb during charge. By hand, I could do no better than getting the cells to about 0.020mv delta before going nuts. Actually, this isn't that bad, but I wanted better, but just could not obtain it.

I let the Optimate do it's thing instead, and bang. 0.005mv difference.

Thing is, you won't think this is happening if you use only the Optimate, as during the cell balancing cycle, voltages are not going to be exactly equal under charge, or even after a few hours rest due to slight differences in manufactured capacity and internal resistance.

BUT, if you then take that optimized battery, discharge it, and then charge it again with a standard CC/CV charger (which I take no higher than 14, and a drop to 0.05C), THAT is where I encountered near perfect voltage balance under charge. I use a Samlex charger for this normal duty - or my solar CC and panel setup.

One tip is that you probably want your cells to be *reasonably* close to each other before the start - that is, if you notice that one cell is surpassing 3.6v when the overall pack voltage is 13.8v, and another cell is sitting at 3.3v, that is way too far off. Use a single-cell charger, or some other technique to get them reasonably close, and let the Optimate finish the job.

So ok, the Optimate is coming from a powersports application (like Antigravity batteries that use A123 cylindricals, and NO balancing circuits), but it worked just great on my larger prismatic GBS lifepo4.

I wouldn't post this if I hadn't seen it before my very own eyes. Remember though that I have only verified balance with 20 and 40ah batteries. I'll have to save up a bit to prove it on something as large as 100ah, which is supposedly the limit for the Optimate.

[Mike90250]

Thanks for the tip.

[PNjunction]

At only 5A, the charger is too small for practical daily use on a large battery, so I'm regarding it as more of an initial balancer, and preventative-maintenance tool.

If I do spring for something like a 100ah lifepo4, I'll most likely be bulk-charging most of it, and then letting the Optimate handle the balancing towards the end. Which in a sub-c application like ours, shouldn't be needed often.

Aside from balancing, it also performs some valuable voltage-retention tests, based on a lifepo4 curve, which take 12 hours or more to determine to help spot radpidly degrading cells or bad infrastructure. It could be quite valuable for those buying sub-standard junk, even though I don't endorse doing that.

I'm really leery of using tools that come primarily from another application, but in this case, it seems to perform the job well.

Years ago, some users of simple 4S batteries that had no balancing boards seemed to notice that the cells seemed to converge on their own, and were summarily trounced on. What was actually happening was that over a period of months of daily cycling, it wasn't the cells balancing each other out, but the repetetive charge to near full that was eventually doing the job. The Optimate just speeds this process along with the rapid up and down cycling near the end.

[Sunking]

how much does it cost?

[PNjunction]

Not cheap! Cost is about $80 to $100+ depending. This is for the 5A version.

It appears they have brought out a much smaller amperage version, the Optimate "4S LFP 0.8A" model. Too small for my use, and apparently incorporates a "bms reset" for those drop-in types. Be sure to get the 5A model for our larger prismatics.

The cost is offset from the typical "hobby charger", even an awesome one like Cellpro Revolectrix, as the Optimate automatically knows that when below 12.8v, it is in the deeper discharge curve, and needs a low-current save until voltage is high enough to use the normal current rates. It also drops current when below 32F or above 113F ambient. Checks for dead cells by following an internal lifepo4 curve, and stops if things are just bananas. Many safety timeouts between modes, and testing both before and after. And of course your battery is naked and proud of it without any hobby charger individual cell wiring and so forth.

You got me thinking - what if we are running a 24v or other voltage bank? You could STILL use the Optimate on your 12v modules and be in balance. How? Simply because the Optimate is not balancing the cells against each other - it is merely ensuring that each cell is truly fully charged with the oscillating charge technique at the end. So buy good quality stuff, and not junk.

So, once you apply the more normal CC/CV charge routine to a reasonably balanced pack, then your bank will now be limited basically by the lowest-capacity / highest IR cell of the bunch. At least this seems reasonable for those being somewhat conservative in a solar housebank setup.

EV'ers like golf-cart racers (ahem) will probably want to bottom balance, so the Optimate would not be used in that scenario.