Eneloop solar charging question regarding partial charging

[eh4]

Is there any harm to charging eneloops partially, then charging them fully later?
Imagine a stretch of days of partial sunlight or overcast conditions, maybe even typically needing more than one day to charge, again and again for the same batteries.

 

[Wrend]

Specifics aside, assuming they were being charged in an otherwise appropriate way, no, not really. But, it doesn't sound very convenient. Maybe if you were stranded on an island with a one cell flashlight and had limited resources and had to make the best of the situation. ;)

 

[eh4]

thanks for the input.
yes, it would suck, but I want to understand how some worse case situations would work out before thinking I'd have a base covered with eneloops and solar.

 

[Lynx_Arc]

I think with spotty sunlight you would have an issue properly terminating a charge and possibly overcharge them. I would say it would work but long term it could degrade your batteries overcharging them if that was the case.

 

[eh4]

thanks for that.
I was specifically thinking of using one of Cotton Picker's usb style charge contollers downstream of the solar panel.

 

[Lynx_Arc]

thanks for that.
I was specifically thinking of using one of Cotton Picker's usb style charge contollers downstream of the solar panel.

If you are going to do that.. make sure and get a panel larger than the minimum you need so you can still charge in spotty sunlight conditions and charge fast enough that you get decent termination.

 

[45/70]

Just thought that I would mention that the biggest problem with only partially charging cells, is that the chances of the cells being evenly charged is pretty much non existent. This may not be a real problem if the cells are used individually, in a single cell light for example. On the other hand, if the cells are to be used in a series application, you may run into trouble.

When cells are charged to completion, in most chargers anyway, they all have a similar state of charge, that being "full". When only partially charging cells, even in a "smart charger", they are likely not going to be charged to the same level. When using these partially charged cells in series, "reverse charging" may occur, which is damaging to cells.

Something to consider anyway.

Dave

 

[pblanch]

I use a Voltaic Amp solar charger and the Eneloop USB charger. 1 full amp battery will fill 2 eneloop AA. This is a very easy, safe and reliable way that I have found, and have the advantage to use the USB battery for anything else I want to charge ie phone. A full day of sun will pretty much charge the whole battery so you have that stored energy when you want.

Should check it out. Additionally the Eneloop USB charger can also recharge individual cells so if you are like me with a ZL its just way too easy.

 

[jh333233]

Dont NiMh still have memory effect?

 

[Wrend]

In way, which also requires knowing what "memory effect" is. Not really an issue for Eneloops though. Still, not a good idea to repeatedly overcharge them, of course.

...And it's probably a good idea to give them a full cycle every once in a while, but I don't think it's all that critical. It may help maintain/improve their performance a bit.

 

[eh4]

thanks guys, good information to consider.

 

[Gregozedobe]

thanks for that.
I was specifically thinking of using one of Cotton Picker's usb style charge contollers downstream of the solar panel.

I thought his chargers were only for 3.7v Li-ion batteries, not 1.2v Nimh eneloops ?

 

[eh4]

good point. what an oversight.

 

[Lynx_Arc]

I thought his chargers were only for 3.7v Li-ion batteries, not 1.2v Nimh eneloops ?

he does sell nimh chargers (USB) but I think they may be dumb timer based chargers (I could be wrong).
Even a dumb USB nimh charger is better than spending a bundle on alkalines and lithium AAs over an extended outage if you have enough of them to warrant a solar charging setup.

 

[jh333233]

Imma explain memory effect, which IS a major problem with NiMH
Each time you charge them, they will have an apparent loss of capacity, which will/can be accumulated until you deep-discharge it
I couldnt say firmly how the charger works but people suggest discharging it then fully charge it, like the mecha auto balance chargers use to prevent the memory effect
Also, i dont own any NiMH anymore so i couldnt tell whether eneloops are superior to old fashioned nimh, im just using the most common senario

 

[filibuster]

Partial charging maybe a factor to be concerned about but the bigger issue to consider when charging eneloops and other NiMH AA cells in a solar application is heat.

NiMH batteries in general fair poorly in hot temperatures and can loose a good part of their cycle or service life (as much as half) when charged or used in high temperature conditions above 113F. In many solar chargers the batteries are exposed to the same heat as the solar panel or even more if the battery compartment is sealed underneath the solar panel which can easily exceed 113F.

I've been looking for a good AA solar charger solution for sometime but keep running into the problems with heat and standard chargers that can miss the minus delta V bump if the sun happens to go behind a cloud when the cells are almost charged and the charger resets and ends up over charging the cells.

In looking at a number of different solar AA solar battery chargers you either have to choose among those that charge at a trickle rate or those that use chargers that are made for a more constant power source and hope they hit the end of charge cycle right. I've also avoided those solar chargers that have the battery charger and solar panel as a single unit which tends to cook the batteries more than others reducing their life cycle considerably.

One AA solar charger that has addressed some of the issue is the Goal Zero Guide 10 plus. I've chatted with the engineers at Goal Zero about their charging method and they've had to make some design decisions and best case guessing to get a good charge into the batteries while avoiding over charging them if the power from the solar panel fluctuates during the charge cycle. The Guide 10 charges the four AA cells in series with a balancing circuit to make sure one cell isn't overcharged compared to the others. And end of cycle is determined by a combination of monitoring the cell voltages and then a timer is started to top them off for a little longer without fear of over charging them. I've been running tests using the C9000 to discharge the cells after they've been charged by the Guide 10 Plus and found that it works pretty well at getting all four cell charged but not overcharged. The C9000 on the same cells would perform better but not by more than a 50-80mAh per cell. Guide 10 plus is a separate charger and can be plugged into the Nomad 7 solar panel via a cable and then placed behind the panel in the shade to give some relief from the heat. If you have a long USB cord you could place the Guide 10 in the shade and let the solar panel sit in the sun. It does charge much faster using a special 6V charging cable hooked directly into the panel. Using USB to charge the Guide 10 is slower because of the low power output of the USB spec.

 

[filibuster]

To address the issue of partial charging directly, if we follow the Toyota Prius charging method then a low state of charge (partial charging) for NiMH batteries is better than a full charge to increase their overall life cycle.

In this article: http://www.cleancartalk.com/battery...ttery-pack-balancing-equalization-monitoring/ it discusses that the designers of the Prius's NiMH battery pack set it to operate normally in the 40-60% SOC to get the best life from the battery pack. If I understand that correctly they are never fully charging their NiMH battery pack nor fully discharging it and because of that they are getting 10+ years of life from a simple NiMH battery.

 

[eh4]

These look like good panels to me: http://www.adafruit.com/products/200
Two or three of them for a folding design. Small dimensions mean less leverage for cracking them should they be crushed in a backpack.
As for heat, with wire leads you could even dig a hole for the batteries to charge in, better than shade in very hot conditions.

thanks for the battery link filibuster.