Easy. There are only two ways to tell how good a battery is:

First, charge the battery and then allow it to stand for a few hours, or place a load on it (like a headlamp) for two minutes to pull down any "float charge" on the battery. (This is a false higher voltage on the surface of the plates, there is no real oomph behind it.)

Buy a good hydrometer (the 12" kind, not the 3" kind!) for about $6 in an auto parts store. Open the battery caps and test the specific gravity in each cell. On a brand new battery this would be 1.265 in each cell.As the cells go down to 50% the specific gravity will go down to 1.190 the hydrometer should have numbers, and a "good-fair-replace" scale in it.

Or buy a digital multimeter (INVALUABLE for reality testing) and test the battery voltage with it, using the 20v scale. You should read 12.75 on that same pristine battery, down to 11.75 on a useless one. 12.25 is a 50% battery.

You want to give the battery a good break before testing it. Make sure they are all fully charged, then top up with distilled water if any cells are low. If you add water, let it sit overnight to mix in and then recharge again. Then take off the float charge as above, and test by your choice.

The hydrometer can spot a single bad cell--which happens. But the hydrometer invariably means a drop of battery acid is going to splash on something that costs more to replace than the multimeter would have cost. (About $20 in Target, etc. for a great deal, about $40 in Home Depot, or Radio Shack on sale.)

Multimeters have a spec sheet with them that tells their accuracy: A real cheap one may have a DC accuracy of +-2% +-1LSD, which means they may be off by 2% of the measurement, plus the Least Significant Digit on the display may be off by one more.i.e. if it says 10.00V, it could be off by 2% of that and the real voltage may be 10.2-9.8volts. But the 10.00 on the meter could also mean 10.01 or 9.999, since the last digit may also be off by one. So...if another ten bucks buys you a meter that is rated for 1/2% DC accuracy, go for it. You can work with either one, you just have to treat it like a navigation position and bear in mind it is never dead on.<G>

It's called a ZAP Stop Alternator Protector and is made by Heart. Heart # 84-6001-00.
Gerry/Mintaka

Jared,
From what I've heard, and experienced( the hard way ), is that switching the batteries while the engine is running, will fry the diodes in the alternator. There is a device that you can get to prevent this, but I don't recall what it's called. Maybe someone more knowledgeable could help us on this one.
Joe

Jared Sherman wrote:
Somewhere sometime I heard that it was bad to mix the batteries (closest to the alternator?) comes up to full charge first, then the other one never gets a full charge. One solution would be to use an A/B switch with field protection. Start on the starting battery only, give it a few minutes to charge, then switch to Both and then to the house battery only. That way the starting battery gets used only for starting, and held as a spare if needed. Nothing needed except the switch and a flip of the wrist. Ideally, if you gang up batteries you want battery cables of identical lengths (i.e. from the batteries to whatever junction point) but the most-discharged battery will be acting more like a short and absorbing the lion's share of the charge, so I'm not sure where the practical reality of that approach will be. I'd be reluctant to be swapping batteries--I hate going near them. Something always manages to get acid eaten!

Properly-charged batteries won't freeze. Those first two words are the trick. Car batteries get charged pretty much daily, at least weekly.

Any lead-acid wet cell battery that is left alone for 30 days will form permanent sulfite deposits in the bottom of the battery and suffer a permanent loss of capacity. After 30 days the loss gets greater. Leave it alone for four months, and you may have a 10-20% loss of capacity. Of course, if you are used to changing batteries every 3 years this may not mean much--but it could add up to a real difference faster than that.

I'm not certain, but offhand I think the loss is 10% per month for a standing lead acid wet cell, so after 4 months you are down 40% of capacity, and the battery is very much more likely to freeze if you have a good February or March freeze.