I have a 12v solar panel for charging batteries, the batteries I'm trying to charge are 7.2v ni-mh, 4500mah - sunshine is not a problem as it will be used abroad, in summer, and I've got all day long
I've tested the panel and it puts out 21 - 24 volts, although the wattage is very low, about 3w. I've charged the batteries with it before, but have been told this is not reccomended, although the plug in charger for these things puts out about
11v, so without spending a fortune, how can I drop the voltage to say, 9v?
All the converters I've looked at online are weird, the input is 12-24v but they only put out 5v.
Will the 23v damage the batteries if I leave it as it is? The plug in charger takes 9 hours @ 500mah to charge one battery, but it's got to be plugged in in the morning and collected at night, but the mains power is a 30 minute journey, hence my need for solar
The panel voltage is completely irrelevant. Once the battery is connected the terminal voltage of the panel will be pulled down to that of the battery. All that matters is the amount of current going through the battery. The only problem with a high panel voltage is that charging will continue even when the battery is fully charged.
However the current limit in your case will be set by the panel. I suggest that you connect the panel directly to the battery via an ammeter* of some sort, as a test. *Or buy a five quid multimeter from Maplin etc. In the UK use a halogen lamp to represent the sun! I think you'll find that the max charge rate will be about 0.15A,which is fine, and when the battery is charged it will decrease somewhat. It will never decrease to almost nothing though, so you won't be able to leave the battery on charge indefinitely. If the panel manages to give a faster charge rate than you'd like use a resistor as a dropper, or put something over part of the panel to screen it.
My own (crude) practice is to charge 6V nicads from a 12V vehicle using a dropper resistor. Find the resistor value that gives a safe charge rate from your solar panel (working at max output) by trial and error. If you know the battery is flat and you will be there to disconnect after a while you can obviously use a resistor that has a higher charge rate. For continuous charge you have to use a high value resistor that gives a trickle charge. This is an electrically inefficient method but that doesn't matter when the power is free. The resistor needs to be a decent wirewound type rated at 5W or thereabouts.
I've charged my 6V lanterns from the van like this for 30 years with no problems. I also occasionally (when the proper charger is otherwise in use) charge 12V nicads from a 30V power supply using a big rheostat as a variable resistor to control the charge rate.
I don't know much (anything?) about the voltage profile of solar panels, but if a 24v panel only produces 3w, that suggests that it delivers a current of only about 1/8 amps (125mA). Presumably you only measured its open circuit voltage, and if you were to impose a load which tried to draw more than 125mA, its voltage would fall? Whether that would allow the current to rise, I don't know.
Anyhow, the only thing that will damage the battery is if you try to charge it at too high a rate. The mains charger charges it at 500mA (*not* 500mAh - that's a capacity not a charge rate), which it apparently doesn't mind. I can't see that the solar panel would have a cat in hell's chance of delivering more than 500mA - so there shouldn't be a problem. the most likely problem is that it may take about 3 days to charge!
If I'm wrong, and there's a danger of too high a charge rate, what you need is a *current* limiter rather than a voltage limiter - but I don't think that's going to be necessary.
I think the point is that you need to use a constant current source and monitor the cell voltages so when its the stable max voltage you just sample charge or do nothing every few seconds. The problem is if you use a simple charger you will run them at too high a current during the charge cycle and overcharge them if you leave it on. Brian
In the main the suggested way to charge these is as separate cells, not in series. The issue of reverse charging occurs in the battery pack charged as they come, as one or more batteries go flat before the others and if you continue to use them this way, assuming the equipment will still run, the empty cell is charged in reverse. So the regime that pays dividends I have found with packs is not actually to use them till the equipment stops, but know how long they will last and take them off just before they run out and charge them with a constant current charger. In many ways they can be seen as supecharged nicads, but the newer ones retain their charge better.
Yes well what I tended to find happened using resistors was that one cell or more died fairly fast. I'm not sure of the reason, but I'd imagine the reverse charging issue when running the gear did something and the crude charger nevetr quite got all the cells up to the same charge in the first place.
Eventually yes. Batteries need to be protected from both being discharged past their point of no return and from overcharging. However at the maximum current your solar panel can supply it will take a long while before problems occur. You are more likely to encounter the opposite problem of the panel leakage stealing charge from the batteries when it is dark if the cheap panel is lacking a diode in series (as many of the cheaper solar power units are).
Until the pressure in the cell becomes too great and it explodes. This is hardly recommended charging procedure with an excessive voltage.
The panel can only output about 3W at nominal 15v or so which is 200mA it might manage a bit more into a dead short but probably not 400mA. That is still below C/10 charge rate so apart from the overvoltage problem your cells could survive on this almost indefinitely.
A simple circuit to regulate the voltage for charging the battery will do what you want. The very simplest would be a resistor to drop the excess voltage when 200mA flows (about 47R and 2W ought to do it).
If you want to be a little bit more elegant you probably should have a diode in series to stop the solar panel discharging your battery overnight (good ones should already have a reverse blocking diode).
The final refinement would be to put a 2W 8v2 or 9v1 Zener diode in parallel with the battery to make a crude voltage regulator. Strictly also a small resistor from the regulated node to the battery as well.
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The circuit is incredibly simple and the bits can be bought from Rapid.
The 5W Zeners are probably better value at 6p each.
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The power resistor still costs slightly more at 7p!
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(expect to pay much more for them at Maplins)
It is only a problem when you forget and leave one on continuous charge from a too high voltage source for weeks at a time. At a C/20 charge rate you will get away with it for quite a long time (especially in this case as they will get a daily rest when the sun goes down).
It depends a bit how you intend to use this device. If you only put batteries in it when they need charging and take them out again at the end of charging (about 24 hours at 200mA so ~ 3 days good sun) then no problem. If you forget about them for weeks on end with a too high supply voltage then expect to kill the weakest cell in the stack stone dead. Eventually it will leak and damage whatever you use it in.
Small solar panel to keep the battery on the genset topped up (in theory). After 6 months connected to panel it was due for a test, battery flat... not amused. Pull starting is possible but hard work (diesel engine). After the test, check for leakage etc in the set none, charge battery, takes charge but one cell might be iffy. Put back into store not connected to panel. This was several months ago, needed it in anger the other day. Started fine, even with it being -3 C...
Thanks to all who replied, given that I have 3 battery packs, all quite old, I'm inclined to leave things as they are, they're only about 13 to replace, if and when one or more need replacing so I'm not overly bothered if one should get damaged / destroyed.
There's no chance of losing charge back into the panel as I'm on the bank (lakeside fishing) all day long and have little else to do other than eat, drink, charge batteries and hopefully catch fish, so they are removed from charge well before dusk, as well as being checked regularly throughout the day. As I stated in my OP, it's worked well and charges the battery as quick as mains voltage, but with the bonus of not having to stop fishing for 30 mins morning and night, coupled with a long walk in the hot sun, so I'm not going to do anything, but maybe on hotter days I'll partially shade the panel as a precaution.
On a related note, don't test a solar panel's output at twilight by using your tongue on the terminal pins, it hurts - christ knows what it would be like at midday.
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