Originally Posted By: Ducman
Perfect response.
I think it could be added, that some standard charging systems on vehicles may not be capable of bringing the battery up to or close to 100% charge in normal service, and are only adequate to keep the starter battery at somewhere above 80% charge.
If possible, It pays to keep the starter battery topped up to 100% with a good quality and properly sized smart charger once a month or so to help achieve optimum longevity and performance.
The capacity of the smart charger is determined directly by the RC of the starter battery.
Yes, Batteries require they be held at Absorption voltages for X amount of time after a discharge. The amount of time varies depending on the amount of discharge, the battery itself, its health, temperature, and the absorption voltage itself.
The higher the voltage, the more amps can flow into a battery, and once the battery is brought upto near absorption voltages, the amps needed to hold the battery start tapering as the charging source regulator is trying to keep the battery from exceeding absorption voltages.
It can be argued that a depleted battery that does not get 14.4v or higher for X amount of time will never reach full charge. I have seen 5 days at 13.6v not fully charge one of my deep cycle batteries. It took another 90 minutes after cranking voltage to at 14.7 before Specific gravity climbed to within 0.005 of the maximum ever observed on this battery.
It is safer to undercharge a battery than overcharge it, so most vehicles will drop voltage to the mid to high 13's well before the battery would actually like. The amps required to hold high 13's are well less than half the amps required to hold 14.6 when the battery is 85% charged, and as such recharging slows down considerably.
If the battery is never discharged much, then mid 13's are fine, when the battery is discharged and all that is allowed is 13.8v, there is not enough voltage potential for higher amps to flow into the depleted battery. So short drives are especially damaging to a car which has sat with the door locks and engine computer slowly draining down the battery.
As a rule, one should never rely on their alternator to fully charge a depleted battery. Ther were designed to power vehicle loads and replace what the starter removes, but whatever voltage regulation is used, is Timid, as timid is safer, at the expense of battery life.
When the voltage regulator does not take into account the battery temperature and raise the voltages accordingly in extreme cold, then very very little current flows into the battery.
Do not be afraid to use a high amp smart charger to get the job. Batteries can easily take tremendous currents when depleted below 80%, and as their voltage rises to the mid 14's the charging sources regulator will back off the current to keep the voltage from exceeding 14.X volts. The charging sources that one needs to be careful with are the unregulated power supplies, the manual chargers, as they, given enough time, can push battery voltage too high, and for too long, and cause battery degradation.
But you can put a 25 amp smart charger on a 50 amp hour battery without fear. The alternator, if allowed to hold seek and hold 14.5 volts, would easily pump twice this much into the depleted battery, and the battery will love it.
If time is not a factor then low and slow can be used, but absorption voltages need to be reached and held before reverting to float/maintenance voltages.
Many 'trickle' chargers will never go above 13.8v and they can take weeks to max out the specific gravity of the battery, if they can do it at all.
So the oft recommended 'trickle charger overnight', is not a good recommendation. it won't do any harm, but it likely will not come close to fully charging the depleted battery, and not do nearly as much good as a larger amp charger that will bring the batteries to the mid 14s for a period of time.
