Most lead-acid batteries will have a "Cold Current Cranking Amperage" usually but not always marked on the outside as "CCA." This is (approximately) the maximum current you can get out of the battery when the battery is fully charged and it's connected to a "standard resistance" that's usually sort of an approximation to a starter motor. The limiting factors that determine the CCA rating of the battery are mostly related to the effective resistance of all the connections inside and between the battery cells.

Although it's meant to indicate how fast you can "take out" of the battery, the CCA rating is a reasonably good indicator of the maximum current you can put into the battery to charge it, although the numbers may not be exactly the same.

Batteries also have a "capacity" expressed in Ampere Hours and sometimes molded into the housing as an "AH" rating. That's the product of the current you drain out of it multiplied by the time it will keep coming. "Flipped around," the AH capacity also tells you how long it will take to charge the battery from "dead" - or very low - to full charge, at a current approximating something in about the same range as the CCA rating.

Most battery chargers apply a voltage around twice the voltage rating of the battery when you first hook them up, but include a current limiter so that the charging current doesn't exceed the rating of the charger. As the battery voltage comes up, the "regulator" in the charger should taper off the charging rate so that the "last few hours" are at a comparitively miniscule current.

If you divide the AH rating (all the juice the battery can hold) by the CCA rating (the maximum current you can force into it at reasonable voltage) you get approximately the minimum time it will take to charge the battery. Since the charger should taper off the rate as the battery starts to fill up, you usually can figure about twice what the simple calculation gives you.

Since most chargers have a current rating much lower than the likely CCA rating of the battery, you need to divide the AH rating by the rated current of the charger to get a reasonable time to charge from that charger - and then multiply by about 2 (or more) since the charging rate should taper off as the battery charges.

A fairly typical auto battery that happens to be out on my shelf has an AH rating or 785 ampere-hours and a CCA rating of 140 amps. From "dead," it would be risky to try to charge that battery in less than 785/140 = 5.6 hours. Actually, since a current that high all the way up to full charge would likely produce a whole lot of hydrogen and very probably an explosion, I'd figure a little more than twice that, or maybe 12 - 15 hours to get to a "nominally full charge." But since my biggest handy charger has a very typical 10 amp capacity, in practice I'd expect about a week to fully charge that one from the charger, if I started from a really "dead" condition.

Fortunately, you don't really need to "fully charge" a battery, and most charging systems, as on your auto, NEVER get very close to the theoretical "fully charged" condition. If a "dead" battery still clicks the solenoid, and sometimes if it only makes a click when you switch the radio on/off, it's still "part way there" so with a reasonably adequate charger a half hour or so usually will bring the battery up enough to start most typical engines. The charging system on the vehicle, once the engine runs, is usually a better way to finish bringing the battery up than even a good charger; although as indicated (sort of) above, you can burn a lot of fuel for a limited benefit if all you're doing is charging the battery. Small generators and mowers may have exceedingly "primitive" charging systems, so you need to study what you've got (preferably by observation of how it behaves) to get a good estimate of how well it "maintains."

When you connect a "dead" battery to a "fresh" battery, the CCA rating of the good one tells you the maximum rate you can expect current to come out of the good one, and the CCA rating of the dead battery will tell you approximately the maximum current you can put into the dead one (with "battery voltage" differences between the two). If you divide the AH rating of the dead battery by the CCA rating of the dead battery, you get a very approximate number of hours you can put current in without getting the dead battery up to full charge. A battery only generates significant amounts of hydrogen when it's "receiving current" and when it's near full charge, so as long as you don't leave a 12V battery connected to a 6V battery longer than the time calculated for the (dead) 6V one, you're unlikely to get much explosive danger. Additionally, once you "hit the starter" it's likely that most of the current that the good battery can supply will go to the lower resistance of the motor, rather than into the dead battery.

The theoretical time it should take to bring the dead battery up to near full charge can give you some idea of how long it's safe to have the two batteries connected for a jump start. Most of the peripheral factors that you'd need to include for a really accurate calculation are "in your favor" which makes the time caluclated as indicated significantly "conservative." With nearly all combinations, it is considered "safe" to jump a 6V vehicle from a 12V one; but it's recommended that you be very conservative about leaving the hookup together if you don't get a quick start, and you should disconnect the jumpers as soon as possible. Left connected for too long, the 6V battery will always start releasing explosive gases after a fairly short time that can cause a "really bad day event."

John