I have an electric fence charger (fencer) for livestock. It has a solar panel to charge the 6V 4.5Ah Gel Cell. But this fencer has not been used for several years, so the battery is completely discharged. It's likely I'll need to replace the battery, but whether the battery is completely discharged, or brand new, I was told the battery should be pre-charged before use, because the solar panel is only meant to keep the battery at full charge.
They sell a charger for these batteries, but from the pictures, it's nothing but a wall-wart, and they want a hefty price for them.
Can I use any 6 volt DC wall-wart? (I think I have a few in my parts boxes).
I also have a small standard 4amp automotive battery charger with a 6 volt and 12 volt switch on it. Can I just set it to the 6volt position and use that? Yes, I know the voltage is correct, but I'm wondering if the amperage is too high for a small battery like this? I was told they are actually lead-acid batteries, but contain th eacid in a gell..... (this may or may not be right).
By the way, I'm no good with math. I know AH means Amp-Hour, but how does that convert to actual amps? I never understood the "hour" part....
Does 4.5 AH mean it puts out a maximim of 4amps for one full hour, or what?
If you're worried about the amps, put a car tail light in series. It's possible that the charger voltage is too high for long-term charging, but should be ok if you turn it off when it's fully charged.
You haven't given us enough information about the capacity of the solar panel to evaluate.
Most "6V" gel cells are closer to 7.2V at full charge. 6V is not enough. Of course if you have an 'unregulated' wall wart it might work so long as you're lucky and its sized appropriately.
If the charger is a dumb "float" charger - it will work, because it should charge a a safe voltage. If it's smarter, it will probably be mostly ok but switch from "fast" charging to "float" too early.
It means, as you have inferred, that if you drain the battery at some constant current "I" amps for some time "T" hours that you should expect to get some useful voltage as long as I*T < 4.5. YMMV!
Charging a new battery before use is a good idea anyway. It was probably given a full or nearly full charge at the factory, but you don't know how long it's been sitting around before you got ahold of it.
Some of the sealed lead-acid chargers are indeed just wall-warts, without any regulation or other control, other than the maximum current they can put out. Some of them are regulated, and some of them even charge in two or more stages, to help speed up charging.
Mouser sells a two-stage one for $30 plus shipping:
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If the maximum voltage it puts out is not higher than the maximum float voltage of the battery, whether the battery is discharged or fully charged, it will work. Also, the maximum current it actually delivers should not exceed the maximum recommended value for your battery.
One manufacturer (Power-Sonic) recommends charging at 7.35 V and at not more than 0.9 A (for your battery), until the current drops to 0.045 mA (for your battery), and then stopping the charge. Alternatively, if you want to leave it on charge "continuously", you can charge at 6.75 V.
If the battery you get is very discharged, what will probably happen is that one of your wall-warts will be under the limits at first, but as the battery starts to charge, it will exceed the voltage limit. If the charge current is fairly low when the voltage limit has been exceeded, the solar panel may be able to bring it up to full. Clean the dust and dirt off the panel and set it outside on a sunny day and hope for the best. (Once the fencer is installed, it's a good idea to wipe the panel off once a month or so, if you can reach it.)
The battery draws what it wants, but the actual voltage output by the charger on the "6 volt" setting may cause the battery to draw more than the 0.9 A limit for your battery. It won't hurt much to try it for a few seconds and see... put a good ammeter (like, built into a multimeter) in series with the output of the charger. Don't rely on the built-in meter on the charger, if it has one.
If the battery charger (or wall-wart) voltage is just a little too high, you could put a single diode in series with the output to drop the voltage by about 1.1 V (for a regular rectifier diode, 1N4001 or 1N5624) or about 0.5 V (for a Schottky diode, 1N5820). These diodes are available from Digi-Key or Mouser for 20 or 30 cents, quantity one.
You might also be able to use a power resistor in series with the battery charger output to tame its output a little, but you'll have to experiment a to find out if this will work. The single filament of an
1156 or 3056 car tail light lamp, or the "bright" filament of an 1157 or
3057 lamp, may work.
I think the first ones available did this. The ones you get now have the acid in a porous fiberglass mat; the term "absorbed glass mat" or "AGM" is probably more correct. If you say "gel cell", everybody knows what you mean.
These batteries are *usually* rated at the 20 hour rate. This means that for a 4.5 Ah battery that is fully charged, you can get 0.225 A from it, continuously, for 20 hours, and then it will be completely discharged. 0.225 A * 20 h = 4.5 Ah.
It scales up and down from this, but the scale is not quite linear. It works in your favor if you hit the battery less hard than the "20 hour" discharge rate. If you draw half the current (0.113 A), you get a little more than twice the runtime (say, 42 hours or so). It works against you if you hit the battery harder than the "20 hour" discharge rate; if you draw twice the current (0.45 A), you get less than half of the runtime - about 9 hours. If you draw ten times the current (2.25 A), you get much less than 1/10th of the runtime - about 1.3 hours.
For batteries like this, I usually order from Digi-Key (Panasonic brand) or Mouser (Power-Sonic brand). I've been unhappy with every other brand that I've tried. I should note that for many years the Power-Sonics I got were built in Mexico, but the most recent set were from China. That set has only been in service in a UPS for a couple of years, so I don't know yet how long they will last. Mostly I use them as replacements in computer UPSes, but some of them have gone in things like portable lanterns.
Even if you don't plan on buying from Digi-Key or Mouser, it is good to get a reference price from them. If you see a seller somewhere else (usually eBay or Amazon) claiming to sell the same brand at way less than the Digi-Key or Mouser price, scroll way down and look at the fine print. It probably says "compatible replacement" or other words that mean you're getting a junk battery rather than the real thing.
If you're ever stuck and need one right now, today, go look in the electrical aisle at Home Depot, near the EXIT signs that they sell. They have a few sizes that are made to go in those signs. A local fire and burglar alarm company will also stock a few sizes, but they may or may not want to sell you one over the counter.
Standard disclaimers apply: I don't get money or other consideration from any companies mentioned.
4.5 AH means that the battery can supply 4.5A for 1 hour, or 2.25A for 2 hours, etc., before a re-charge. 'AH' us Ampere-Hour and is a measure of battery capacity. A charger should be able to supply a maximum of 450 mA. Why not just design and build a simple linear power supply for the fence, from COTS components, using the rated current/voltage for the fence. Hope that helps.
You can buy smart chargers online. There are essentially two types of charging:
Cycle : Apply 6.8V If current is drawn, charge with limited current to
7.2V until current falls to 45mA. Return to 6.8V. Charge will reach nearly 100%.
Float : Hold at 6.9V. Charge will reach 70% then very slowly climb to
80% or more.
Solar charging with the fence always on would be similar to cycle charging since the battery will only approach full charge near the end of the day. Just shunt the voltage to 7.2V.
If the fence is not always on, shunt to 6.9V.
4.5 AH for gel cell means that it should average 225 mA over 20 hours.
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