How to calculate maximum dumb charge voltage and ampage?

[wingnutLP]

Hi,

I have a battery or in my case several batteries 3 x sanyo 2700 mAh aa NiMh in series.

How would I work out what is the maximum voltage, ampage or wattage I can dumb charge them at and which of these is the limiting factor?

I hope that question makes sense and apologies if it is a dumb one! :sigh:

I am trying to size up a solar cell for dumb charging them and I am not sure which is the important number. I assume Watts but it doesn't seem sensible that I could use a huge voltage and low amps!!

Thanks for yor help!

 

[wingnutLP]

I think I have answered my own question here!

10 hour charge is generally accepted as conservatively OK for a trickle charge so if I have 3x 2700 mAh cells that is a 8100 mAh battery.

I think therfore that I should be able to charge it with a pannel that has an output of 800 mA.

Is that correct?

Does it matter what the voltage of the panel is?

This thread is awesome by the way: http://www.candlepowerforums.com/vb/showthread.php?t=217683 thanks to TakeTheActive


Edit***

I am a muppet, it can't be the Amps that are the limiting factor, it myst be the wattage of the panel.

So can someone please tell me if the way I would work this out is to calculate the wattage of the battery pack and then use 10% of this to charge the battery?

Thanks!

 

[SilverFox]

Hello WingnutLP,

If you have 3 cells of 2700 mAh of capacity in series, you end up with a battery that has a voltage of 3.6 volts and a capacity of 2700 mAh.

NiMh charging is current controlled, so you would want to limit the current to about 250 mA, and you would charge for about 16 hours.

In addition, you want to limit the maximum voltage of the pack to under 2 volts per cell (or under 6 volts for a 3 cell in series pack).

Tom

 

[hopkins]

I've got a Sunlinq 6.5 Watt Solar Charger panel
-the kind that folds up-
Just checked and it charges 3 NimH AA cells in series
at 360mA. the individual cells measure 1.4v under this
charge. (half discharged state at the time of measurement)

So when the cells are full and if I let them continue to charge
at this rate they'd heat up and suffer reduced life.

A thought: as the sun moves across the sky place some object
that will throw a shadow across the solar panel to stop the charge
after they are full. Need to estimate the previous discharge state
charge rate from the panel at various sun angles, hours to charge to full.

Not sure if you'll be leaving the panel unattended or if you'll be able to
set a timer and then go outside and unplug it to stop the charge cycle.

A bit of fun

 

[J_C]

Seldom do people get the max rating out of their solar cells. You don't necessarily need to limit peak voltage to 2V per cell, tons of times I've charged pairs of AA off a 12V wall wart with a series resistor and no problems over many years. Generally speaking you want a solar cell array with a peak output voltage rating at least double the peak charged voltage of your pack, they don't behave like a current @ voltage regulated power supply would, the voltage drops like a rock at higher current levels. For 3 AA that's roughly 3 * 1.45V * 2 = 8.7V. The current of the cells depends on how fast you need it to recharge and how often it's used, generally a dumb charger may not completely recharge the cells fully, you pick a cell capacity above what you actually need in capacity per day and they were perpetually between empty and full charged. I agree about the general concept of a rate for 10 hour charge but you won't get 10 hours of sun continuously at the same strength to have a constant current from the solar cells, what you'll need do is pick solar cells that don't peak at over 1C charge rate at the very least then take some measurements in the environment you'll use them in, the longest days of the year with least clouds which isn't going to be right now so for a build this time of year you'll have to be conservative and expect higher charge rate in summer. However, NiCd would probably work better for this application, better cold weather performance and more recharge cycles despite being lower in capacity, though the new Eneloops look promising with their improved # of charge cycles but I don't know if they've hit the retail stores yet.

 

[J_C]

There is another option, albeit a more expensive one. Ok, two options with the first obvious one being a solar charge controller, but the second being to use a regular NiMH battery charger with a 12V DC input jack, then plan your solar panels around roughly 18V peak, 0.5A. One virtue of doing this is that lots of things run off 12V too, it can be used for more than just charging the batteries though you didn't detail the goal/use of the project.

 

[wingnutLP]

Hello WingnutLP,

If you have 3 cells of 2700 mAh of capacity in series, you end up with a battery that has a voltage of 3.6 volts and a capacity of 2700 mAh.

NiMh charging is current controlled, so you would want to limit the current to about 250 mA, and you would charge for about 16 hours.

In addition, you want to limit the maximum voltage of the pack to under 2 volts per cell (or under 6 volts for a 3 cell in series pack).

Tom

Thanks, would the output of the panel be 250mAh as it is about 10% of 2700 mAh?

So the "rules" are about 1/10 of the mAh of the battery and 2v per cell as the limit?

This will be unattended charging. I am making a budget LED lighting system for off grid use; the batteries will power an XP-G run direct through a resistor. I am aiming for about 3-4 hours light per day.

What would be the cheapest solar charge control chip or board? I want to keep this as simple as possible so if I can dumb charge and dumb light I think that would be better.

That said I do reaalised that Dumb charging NiMh is not great and NiCd are really hard to get in AA format. I want to keep it AA so that if necessary the batteries can easily be replaced or at a push you could use primaries.

Thanks for all of your help

 

[J_C]

Here are some NiCad D cells that seem somewhat suited to the project,
http://www.allelectronics.com/make-a-store/item/NCB-4500/NI-CD-RECHARGEABLE-D-CELL-4500-MAH/1.html

Remember you will not get full rated output from the panels most of the time, you have to derate to the average wattage the panels will produce per day and compare to the driving current you want to set for the desired 3-4 hours runtime.

Solar panels don't behave like a regulated power supply does, more like an unregulated wall wart. You absolutely MUST pick one rated for a higher peak/unloaded voltage of over 2V per battery cell or you'll never get enough charge remembering that is only the peak output, brand new cells, clean, cloudless day, etc. .If you are fortunate there will be secondary specs like current @ (lower) voltage, but modular may be the way to go, start out with a 5 or 6W panel and if you find your location, positioning, season per this time of year/etc, isn't enough, add more panels till you measure it reaches full charge.

One thing is for certain, without a charge controller part of the time, the peak daylight hours every day it will need to charge at higher than C/10 rate if you expect to be able to fully charge and discharge the battery pack daily, but other times of day it will charge closer to, and less than this rate. With a charge controller you'll need even larger solar panels but will have more assurance of reaching a full charge without damaging overcharges.

 

[wingnutLP]

It seems that you can't buy NiCd batteries in England and possibly any of Europe any more. I think they have been banned due to the toxicity...

How about a small lead acid battery running at just 4 v. I know that it would not be interchangable in teh same waay an AA or D cell would be but the chemistry is probably better suited to this type of application?

The other option is NiMh and aa charge controller I guess?

Or maybe a big NiMh battery dumb charged so that it never reaches full charge? But this option seems a bit wasteful...

 

[J_C]

Lead acid has significantly fewer discharge and recharge cycles, would wear out much faster unless a substantially larger battery capacity were chosen such that it never drained a high % of it's total capacity before being topped off again from the next day's charge.

For some purposes that is ideal, to maintain a permanent, perpetual reserve capacity energy store at lower cost per stored AH, like if you live off the grid entirely, but unless there were already a largish spare deep cycle SLA battery lying around it seems overkill.