24v Nimh battery issue

[robotcoder]

I bought a 24 volt battery pack a few years ago and haven't had any issues with it until today.  I went to check the voltage and it showed 1.2 volts.  I thought maybe there was a short, but Everything looked fine with the connections, any ideas about what happened.  Is the battery pack bad, or should I just try to charge it.

[waltr]

You can try changing it but carefully monitor the temperature of each cell.

If the pack does not take a change you can then try checking and changing each cell. You probably need to remove the covering to access each cell. Try bringing each cell up to charge, discharge it some then try charging the whole pack again. This is called "balancing" the pack and you can find more info about this on the web.

NiMh batteries do self discharge so leaving them sit for any length of time is not good. What then happens is that one cell will discharge more than the others then this one cell kills the pack's total output.

[johnwarfin]

as usual i have a couple minor corrections:

repeated charge/discharge to restore nixx is called "cycling" not "balancing". balancing is something only done for lipos and even then is rarely needed. cycling has little benefit for lithium type and in fact only serves to reduce life expectancy.

full discharge does no harm to nixx. in fact it is part of the restorative cycling described above. surplus cells from nasa programs are often supplied with dead short accross the terminals. over-discharge does have detrimental effects for lipos but nowhere near as bad as commonly believed.

[waltr]

Thanks John but the 'balancing' is the process on changing each individual cells in a pack so that they are all the same or balanced. NiCd and NiMh packs can have cells that are not matched and therefore require 'balancing' to restore the packs capacity.

[Roman505]

True.

Given temperature was mentioned, we should put some numbers on it for robotcoder. I suggest that the pack temperature not be allowed to exceed 45°C. If inter-cell variations are greater than a few degrees C then discharge the cells individually as suggested by waltr to a uniform individual voltage then try the pack again. One cell increasing in temperature significantly beyond the others can indicate a dud cell.

[johnwarfin]

Thanks John but the 'balancing' is the process on changing each individual cells in a pack so that they are all the same or balanced. NiCd and NiMh packs can have cells that are not matched and therefore require 'balancing' to restore the packs capacity.

this is correct only in the most general sense. its true that both types need to have cells equal charge. but in battery technology the term "bablancing" usually applies only to lithium chemistry. lipo charging sometimes involves extra balancing leads but nixx charging does not.

the term for bringing nixx cells in a pack up to equal state is generally refered to as "pre-conditioning" by engineering professionals and experienced hobbyists. its true that the goal is to get cells to similar levels but a completely difference process is used. nixx are "balanced" by trickle charging a pack. usually done when new or sitting for a while. otherwise peak (delta) charging is the norm for both nicd and nimh.

on the other hand lipo balancing is accomplished with a much more complicated circuit specific to the function of equalizing cells. unlike nixx, lithium cannot be trickle charged. they will self destruct if over-charged unlike nixx which tolerate considerable over-charging. cc/cv (constant current/constant voltage) is the normal way to charge lithium and true balancing is often involved.

to recap: nixx are "pre-conditioned" by trickle charging. lithium batteries are equalized with special balance leads that are separate from the high current charging leads. balancing leads are never used with nixx. they self balance via the charging leads.

another interesting difference is that nixx cannot be charged in parallel but lipo can. very safe to charge nixx in series but dangerous with lipo (w/balancing circuits).

ive been designing battery power circuits and chargers professionally for some years now. if you like i can go into considerably more detail on the two basic types of lithium balancing circuits. also a lot more on the nixx methods if you like.

[johnwarfin]

One cell increasing in temperature significantly beyond the others can indicate a dud cell.

if you are talking about nimh, i disagree. within limits heat is a normal part of the process and occurs when fully charged. in fact it is the underlying principle of peak/delta charging: heat=voltage drop=stop charging. at really high currents too much heat can cause electrolyte loss. thats why "fast charging" is a bad idea if you want cells to last.

[Roman505]

I am not seeing that you are disagreeing with me, John, but rather not including the same distinctions. I covered the question of heat increase and gave the limits I used for RC car race packs (45-47 as one of the safety measures alongside the primary test of V-drop sensing) so of course I know temperature rise is an expected part of the process. However, if one cell is heating much faster than others in the pack then the implication is that it will not achieve an energy density comparable with others in the pack, peaking (and thus discharging) much earlier. In a racing car pack that is unquestionably a dud cell. General heat rise and differential rise have different implications.

ETA: OF course, I must allow that one cell reaching charge faster could imply it simply started at a higher voltage, which is why the basis of balancing NiXX is individual discharge to a uniform voltage before you can test properly their charge rates.

[johnwarfin]

yes, reviewing your statement "temperature significantly beyond the others can indicate a dud cell" i see its true. bad cells will heat sooner due to the lower capacity. also during discharge due to increased internal resistance. i guess i was implying that one cell in a good pack also starts heating sooner because capacities are never perfectly matched. but if one got hot a LOT sooner this would be cause for concern.

and your temperature references do seem in line with regular safety measures. heat is the real enemy of all chemistries but hobbyists dont always appreciate this. pushing their packs to the limits until its too late.  amazingly many dont take time to find out what c rating means or even own a meter.

[johnwarfin]

oh yeah, right, if they dont all start from the same state of charge too. an option to fully discharge is a feature of "good' nixx chargers. i notice with my 10 bay tenergy one or two cells always get hot to touch before the others if i dont take advantage of this. i started marking them to track this which showed fortunately it was not the same ones every time so... all good.

cycling is another useful feature to restore nixx suffering from "memory". contrary to popular belief cycling serves no purpose for lithiums and only shortens life.

[Roman505]

Interesting to read that about lithiums. I have little prior experience of that chemistry apart from the batteries in my computers. Is it sound that they should be charged (and possibly discharged) thoroughly the first time(s) but after that they are fairly tolerant, or does even that not matter?

[jwatte]

Computers or cell phones should be charged to full when you receive them, because they will likely be at half charge or less (best storage level.) You should not fully run down a lithium battery, unless you need to because you're not near a power outlet :-) You should not "dwell charge" Lithium once it's fully charged, although good battery management circuits take care of preventing that for you.

Even "full discharge" typically means 3.2V per cell, which is a cut-off that prevents discharge damage to the cell, but there's a small amount of energy left in the cell. This level is enforced by battery management circuitry, not the cell. Thus, you must always insulate the terminals of a Lithium battery when you transport/ship it -- shorting a "dead" cell may still cause an accident. By contrast, various NiMH batteries are often shipped fully shorted.

[robotcoder]

Thank you for the replies, they have been helpful.  I called the support line for the place I purchased my battery from and they said that in order for my Smart charger to begin charging my battery, my battery needs to be the minimum voltage (18 - 24 volts).  Does anyone know a way I can Dumb charge the battery to 18 and then Smart charge the rest of the way.  I only see smart chargers for sale.

[Roman505]

If you have a genuine smart charger then add batteries, sit back and wait for full charge.

Exactly what charger do you have?

If you want to deal with the first part manually then put the battery on a constant-current charger limited to 1/10th of nominal capacity, and leave it there for 15 hours then take it off.

[jwatte]

robotcoder, we still don't know what chemistry your battery is. How (and if) you recover it varies based on the chemistry.
Is it sealed-lead-acid? NiMH? LiFePO4? NiCd? LiPo? Something else?
(EDIT: I just saw that the subject of the thread says NiMH.)

If it's a battery that's "safe" to bring back from the dead, then the best way to dumb-charge is as said: supply a constant-current max-voltage power source, such as a benchtop DC power supply, and trickle in the juice. For example, if your battery is 24V, 5 Ah, then set the voltage limit to 24V (nominal) and the current limit to 0.5A (1/10th capacity or 0.1C.)
Even with "safe" chemistries, you want to check for overheating, cell swelling, or other possible hazardous conditions while pre-conditioning them to avoid a chemical explosion or house fire.

Lithium-based chemistries are generally not possible/safe/worth-it to bring back from the dead.

[johnwarfin]

ops comment that the charger will not work if cells below a minumum indicates lithium chemistry. as you said in your ecxellent summary nixx can be 0v and any charger will bring them up from there. if they are lithium then at 1.2v sounds like his pack is damaged or dead. not worth the risk to attempt recovery.

btw any cheapo wall wart can be used to trickle charge nimh. its also a trick to get lithiums up to a voltage that the charger will accept. VERY dangerous though and not for beginners. even those with experience would be foolish to try this w/o a meter attached or unattended.