Re: Automatic
I have three of the little buggers and as they began to proliferate around here, I extended the wires with ring terminals that went to the batt and put a small set of batt clamps on so I could easily swap that one charger from batt to batt. Next I think I will install a polarized socket on each vehicle/thingy I want to charge so I can just plug the charger in at an acessiblel location and not have to raise the hood or climb under my dune buggy. Mine cost about $30 and are designed for permanent installation into a vehicle (poo to that!).
I have one on my mom's deep cycle battery for emergency lighting and power for her safe room. So far I have been pleased with these units (Shrader).
Someone asked what could go wrong with this sort of item. I won't attempt to address all the failure modes but I will describe a way for a unit to be designed/implemented incorrectly. First, let me say that charging a lead acid batt can cause it to outgas via electrolysis (consuming water from the electrolyte). This is especially true when the batt voltage is high and charging current continues.
Often in a batt charger, a regular step down transformer like you might use in a power supply has rectifier(s) in its output to make pulsating DC to charge the batt. The RMS voltage of the waveform might be equal to the float value selected, say 14.2 volts. If the RMS is 14.2 volts then the peak is about 20 volts. During that part of the alternation when the output is between 14.2 and 20 volts the battery continues to charge some. This sort of circuit won't stop charging until the batt reached 20 volts and as that won't happen the batt gets overcharged and in the process gets "boiled dry" (actually looses electrolyte through electrolysis). A smarter circuit uses a SCR, triac, or whatever to turn the current off when the batt reaches the set value. Usually there is a little hysteresis designed in so the batt has to fall down a bit in voltaage before the ckt turns back on (like a low pressure turn on and high pressure turn off for a water well controller). Sometimes there is a LED that is on when charging and it will begin to blink rather than be on or off when the unit is cycling at the set point.
There are a lot of different ways to make one of these chargers that works without hurting your batt. As you employ a new one (a brand/type not familliar to you) monitor the electrolyte level and batt voltage frequently. If it doesn't overcharge voltage wise or cause the loss of electrolyte you can safely lengthen out the intervals between voltage/electrolyte checks. I would be happy to see the voltage level out at a value greater than 12.6 and would get nervous if this sort of charger got over 14 volts or so. I trust the units I am using, based on actual experience, to be safe to ignore for a few months at a time except maybe on the
chipper batt (smallest one I have but havent used it there yet). Like others have said, I rotate one of the chargers between a couple three different vehicles depending on usage patterns.
Regarding the attached sketch: "A" is the positive alternation, "B" is the negative alternation, "B prime" is the rectified negative alternation. The dashed horizontal line is the DC voltage level equivalent to the rectified waveform compried of A and B prime. If the DC equivalent value shown by the dashed horizontal line (AKA RMS-root mean square) is 14.2 volts then the peak value of the two positive pulses is nearly 20 volts. Difficult to see in the drawing since I scaled it to show that 10 volts RMS is about 14 volts peak. The ratio stays the same with a sine wave voltage. The peak value is 1.414 times the RMS value. Battery chargers usually do NOT have filtration and their outputs typically are NOT smooth DC. So, should you venture to Radio Shack to get parts to build your econo-charger be careful that you don't overcharge your battery and dry it out. This can be done over time even with a charger that is capable of less than 1/2 amp of current, provided that it isn't designed properly. the quick and dirty charging circuits are fine for temp use but should not be left connected indefinitely as they will ruin a batt.
Patrick
