Is it possible to 'top-up' eneloops or other Ni-MH batteries?

[Ohmic]

Hello all,

What are people's thoughts on 'topping up' Ni-MH cells?

For example, can eneloop cells be charged back up to full capacity, if there is substantial capacity remaining in the cell?

There are times when I may have cells which are at 50-75% capacity, and rather than fully discharging, then charging again, I have questioned if it ok to just put them in the charger and top them up.

Back in the day when I raced RC, common practice was to always completely discharge the pack (7.2V SubC Ni-Cd) down to 0.9V per cell, then once cooled down, hit it with a recharge. We never dare charge a Ni-Cd pack unless it was completely flat first.

BTW I have the Maha MH-C9000 for charging duties.

Thanks everybody:wave:

 

[jasonck08]

Ni-mh cells do not suffer from the "memory effect" in the same way that Ni-cd cells did back in the day. It is perfectly fine to top off Ni-mh cells.

 

[McAllan]

NiCd didn't suffer very much from "memory effect" either.
It requires quite specific conditions for that to occur.

Mostly the cells where just being ruined by bad chargers and neglect from consumers who let them cook for days in a bad charger.

 

[TakeTheActive]

...What are people's thoughts on 'topping up' Ni-MH cells?

For example, can eneloop cells be charged back up to full capacity, if there is substantial capacity remaining in the cell?

There are times when I may have cells which are at 50-75% capacity, and rather than fully discharging, then charging again, I have questioned if it ok to just put them in the charger and top them up...

It depends...

1. How often do you recharge your cells?
   .
2. Do you ever discharge your cells to ~0.9VDC inside their devices?
   .
3. How well does your charger terminate?
   
   Experiment: Is Your Charger Terminating Properly?​

The example I like to use to answer questions like this is 'Charging your cells once-per-week for 10 years will be 520 cycles'. Throw in a 'REFRESH & ANALYZE' every 10 cycles and a 'BREAK-IN' every 30 and you're good-to-go.

But, if you're recharging less, then your cells are spending more time 'Self-Discharging' than working and you need to adjust. For the cells that I recharge every ~1-4 months, after I remove them from their device, I DISCHARGE them on my BC-900 @ 100mA. Then I use either my C9000 or my BC-900 to run a REFRESH @ ~0.5C/0.2C.

You need to 'exercise' ALL of the chemicals in the cell on a regular basis - either WORKING in your devices or CYCLING on your SMART Charger/Analyzer(s). You also need to monitor their Internal Resistance. If it's rising, possibly you're not 'exercising' ALL of the chemicals and crystals are forming.

......BTW I have the Maha MH-C9000 for charging duties...

Good choice.

Personally, I think that the C9000 and the BC-900 compliment each other - each has strengths and weaknesses - together they pretty much handle my AA/AAA cell inventory for their lifetime - from VIBRANT to CRAP to HIGH to MODE to TRASH.

CLICK on my Sig Line LINK for lots of interesting Newbie 'Questions & Answers'.

 

[McAllan]

Don't know if anyone really has to speculate all those numbers about discharging fully at x interval and break in at x. As long as cells are just discharged fully once in a blue moon.
And with LSD cells it seems even less critical.

It's quite fine to top up both NiMH and NiCd no matter if they're 80% or 20% full.
One warning though that is especially critical to an LSD cell and that is not to top up a cell which is already full. Many cheaper chargers will then miss termination. I haven't tried with the C9000 but I suspect it might miss it.
My older Conrad Charge Manager in contrast is very nice. You can put in freshly charged cells and it'll stop after very few additional mAh and in the case of LSD cells after a few days it'll stop after typically 24 mAh. In English it means practically no overcharge - and way lower than the cells are designed to handle without problem.
So often use the Conrad if in doubt whether the cells are charged or by how much. If they are no harm is done - but as said you really need to be sure your charger can handle the situation. Otherwise it's best to discharge them a few 100 mAh in the case of AA and then top up if you need them fast.

 

[Ohmic]

It depends...

1. How often do you recharge your cells?
   .
2. Do you ever discharge your cells to ~0.9VDC inside their devices?
   .
3. How well does your charger terminate?

1. I'd say the longest they go without charging would be maximum 2 weeks.

2. I always take the cells out when the light I am using them in goes noticeably dim. (Fenix LD20 and TK40).

3. I am not sure, but I have had no missed terminations yet. I like to charge my cells fast, and charge them at least 0.5C, sometimes even 1C.


The reason I ask about the 'topping up', is because there are sometimes nights we go out with the flashlights for a couple of hours and I want each cell to be fully charged. The reason I ask about topping up is because before those big nights, I use the flashlights intermittently throughout the week.

Discharging 8AA batteries completely, then recharging them is a time-consuming exercise when you're charger accomodates 4 cells at a time. I estimate the cells would have 50-75% in them when I would top them up. Plus I wouldn't top them up all the time, probably once every 2 or 3 complete charge-discharge cycles.:thumbsup:

 

[Bones]

Actually, the MH-C9000 is especially suitable for topping off premium quality cells.

It terminates its primary charge at 1.47 volts, so as long as your cells can accept a charge beyond this voltage, it should terminate reliably at virtually any of its rates. Eneloops, for example, routinely reach 1.56 volts with a charger that relies on negative-deltaV (-∆V) to terminate its primary charge.

I would also recommend leaving the cells in the charger for at least two hours after the primary charge terminates to take advantage of the MH-C9000's 100mA top-off charge. If you monitor your cells towards the latter portion of the two hour top-off charge, you can actually see whether they're having any problems charging beyond 1.47 volts.

With cells that can't charge beyond 1.47 volts, the MH-C9000 relies on -∆V to terminate its primary charge, so these cells should be charged at a rate high enough to maximize their voltage drop once they've reached an over-charged state.