Charging NIMH (and most modern battery chemistries) isn't done at constant voltage, but at constant current.
You'll need some fairly involved electronics to provide a constant current source and monitor the battery state.
Battery University has a pretty good description of the charging cycle of NIMH batteries.
The initial charge is at 1C (in your case, 175 mA). There are 3 different stop-checks (whichever is reached first):
- Temperature rise rate of the cell being charged.
- Voltage across the cell.
- NDV (negative delta voltage). Basically check for when the voltage rise of the battery stops rising at a certain rate.
At this point, the charger will charge the battery at significantly less current, repeating similar 3 checks and iteratively stepping down the current charge rate.
The linked page suggests that slow-charging a NIMH battery is difficult/impossible because it's very difficult to get reasonable readings for many of the time-dependent stop checks. You're basically relying on a time charge, which could excessively charge the NIMH cell and damage the cell.
The primary safety concern is excessively heating up the battery. You might risk rupturing the battery, though I don't think NIMH are susceptible to catching fire as some other cell chemistries are. Most likely the battery casing is designed to rupture in a safe manner, but no guarantees.
It's unlikely you'll do anything which would be considered a significant safety concern, just use your common sense. There aren't any high voltages involved, and the power outputs are relatively low.
Measuring the battery voltage "as received" prior to charging "is always wise"
However, this is a scam.
Battery
The battery contains 3 x 3.7V cells (nominal) rated at 1380 mAh each.
Placing 3 in series would at best give you a 11.1V x 1380 mAh battery.
IF they had been in paralle it would nominally be a 3.7V x 4140 mAh battery
So the 12V x 3000 mAh claim is spurious.
A LiPo cell has a
maximum voltage of 4.2 V (So 3 x 4.2 = 12.6 = Vmax_charged)
an average voltage over the whole discharge of ABOUT 3.6V or 3.7V / cell
and a minimum voltage of about 3V/cell.
Discharge tyo much below 3V/cell is liable to damage the cells and discharge to 2V/cell will fatally damage the cells.
So not only did they sell you fake junk, they sold you dead fake junk :-(.
There are many of these for sale on the internet at various prices.
IF you bought one from Seeed studios or Deal Extreme you could very likely get your $ back.
From elsewhere YMMV.
Dead Jim You Tube.
Ali Express $13.80
DX.com - capacity claim 1500 mAh V still suspect <- Somewhat reputable seller.
Amazon $31.80 junk
Amazon $28 whatever
Bang good $18.10 low price junk
Fasttech $16.45 even cheaper junk
:-(
ADDED 1/2016 in response to reader query.
How is it fake? I have a couple of this exact same battery, and they work fine and they each report 12.6V when fully charged, from the 3 x 4.2V cells. That's how you measure voltage in series.
Read the above explanation carefully and you should see where the claims are wrong. The 3 x 4.2 = 12.6V is indeed what he should see when fully charged - that's what you get, which is good. He gets ~= 2V which is fatally bad.
Even if the 3 cells in a 3S pack were 500 mAh, or even 50 mAh capacity, they would still measure 12.65V when fully charged, so battery voltage within range tells you very little. Battery voltage outside range tells you much.
Vbattery
over about 12.8V is very bad and
under about 9V is also bad.
under about 8.5V is possibly fatally bad
(2 x 3V marginal and 1 x 2.5V death's door).
under about 7.5V is junk.
When you place cells in series (3S here) the voltages add (as you say), but the mAh of the combination is that of the lowest capacity in the string - in this case 1,380 mAh for THE WHOLE string as they are all have the same capacity = 1380 mAh.
LiIon and LiPo cells have a mean operating voltage of around 3.6V to 3.7V and this is whyat is usually used for quoting battery voltage. Vmax is 4.2V/cell and SOME manufacturers use this voltage when specifying battery voltage, but this is a marginal practice - anyone who does it without comment is quite likely to be "cutting corners" elsewhere.
If you add batteries in parallel (3P if you did that here) (they must all be the same voltage) the mAh ratings DO add but Vout is V for one battery. So his battery could legitimately be described as
[10.8V, 1380 mAh] (at 3.6V/cell), or
[12V, 1380 mAh] (at the marketing hype 4.2V/cell) OR
[3.6v TO 4.2v, 4140 mAh]
but NOT [12V, 3000 mAh].
That is what makes it fake.
IF the cells used are in fact 1380 mAh (and they may be lower)(ask me how I know*) then capacity = 1380/3000 = 46% of claimed. That's fake, alas :-(.
IF your version has 1380 mAh cells the same applies.
The battery cam be 12V, 3000 mAh with 3 cells ONLY if it contains at least 3 x 3000 mAh cells.
If readers disagree with this or do not understand it please do comment or ask - the aim here is to learn.
- I've spent 6+ months in China on over a dozen electronics manufacturing related visits in the last decade - which has taught me a lot about what you may expect worst case. China can and does make excellent products. But, also, extreme rubbish if uncaring profit focused and/or ill informed resellers let them get away with it. Unfortunately, many do. Almost no specification or claim is trustable if the reseller does not stand behind it and has not done enough "due diligence" to be certain that their supplier is reputable and reliable.
A very large proportion of product from "no name" manufacturers or suppliers does not meet claims and/or specifications to some extent.
Best Answer
Yes, due to chemistry losses given out as heat.
Yes, provided the charger has enough "headroom" to drive 300 mA through the batteries as their voltage will rise during charge.
The charger must be capable of a higher voltage than the batteries if current is to flow from the charger into the batteries. For example, your car alternator gives out about 14 V to charge the 12 V car battery. (Be careful though: your car is using a lead-acid battery which is charged at constant voltage whereas your AA cells require constant current but still with higher voltage.)
As discussed 6 V might not be enough. Even if it was, at 100 mA charger current and, say, eight hours good sunshine per day it will take at least three days to charge your cells. If you have to split them into two 3 V pairs on account of the voltage you might only get 50 mA per circuit. Charge time will be six days.