landyacht318 wrote:
4x4van wrote:
My cheap HF charger (2/10/50 Amp) tapers down the charge amperage as the battery SOC increases. I've never used it in an RV for an emergency power supply, but wouldn't be worried about it overcharging. Not sure why I would need a "timer", as it automatically ramps down to just a trickle charge.
This statement reveals an incomplete understanding of how batteries charge or how battery chargers work.
The charger has a maximum allowed voltage.
As the battery nears full charge it requires less and less amperage to maintain that maximum voltage. This is called constant voltage stage or absorption, whereas the time before battery voltage has reached that plateau is called bulk, and is a constant current stage.
Float stage applies just enough amperage to maintain a lesser voltage that will not overcharge the battery. Most garage 'smart' chargers in float stage will have a hissy fit if the current required to hold float voltage, or absorption voltage varies, and it will vary if someone is inside turning DC items on and off.
So yes, the charger ramps down the current, but not because it senses the battery is nearing full, it simply is ramping down as it it trying to prevent the voltage from going higher than the maximum allowed. If the current were constant, not allowed to taper, then battery voltage would keep rising, and 12 amps constant on an otherwise healthy group 31 flooded battery can likely push voltage up into the 17's in just a few hours after current would have started tapering had voltage been capped at 14.7v.
All the while the battery fizzes up a storm, smells of sulfur burps and spews electrolyte, and begins heating up rapidly in thermal runaway. Those Giuant wheeled chargers at auto repair joints from 20 years ago can easily accomplish this if they are left on for too long.
As to when a charger shuts off, well this amount of time it holds absorption voltage, is a guess, as every battery will be different and change as they age, so the time in which to hold absorption voltage is a big guess, with a large lean toward safe, which means undercharging as overcharging can be dangerous.
The battery that has a chance to actually achieve full charge, but is not allowed the time to do so by a 'safe' charging source, is like getting kicked in the balls right before climax.
The chargers which insist that amperage taper to such and such a level before dropping to float, when used as a power supply in an RV powering loads, might possibly keep the battery at 14.7ish volts long after it is fully charged, so NOT worrying about it overcharging in such a task, could be unwise.
How much effect it will have on the battery if this is to occur, is highly variable, if the battery is sulfated then the prolonged time at 14.7ish volts could be beneficial. If it is healthy and then held at 14.7 for many many hours afterwards it is being overcharged, using water, and shedding positive plate material.
Anybody using a regular car charger as a power supply would be quite foolish to not monitor battery terminal voltage initially and then later on, and every so often thereafter. That is if the charger remains on and does not shut off thinking something is wrong.
Never assume it will be 'just fine', unless you have experience with that specific charger in the same specific usage on a battery at and battery capacity at nearly the same state of health.
'Just fine' could be uttered a few seconds before something is just about to fail, without any tools or data or previous experience doing the same thing with similar variables to make such a declaration.
I equate proclamations of 'just fine', in most cases, to be words uttered by the deliberately and proudly ignorant.
Interesting. So you're saying that even though the amperage tapers down on the charger, the voltage stays high, and could push the actual battery voltage way above safe levels if left on too long? I have in fact left my charger hooked up overnight AFTER the meter has tapered down to zero and the green "battery charged" LED comes on, but I've never actually checked the voltage that the charger is putting out, neither when first hooked up nor after it has tapered down; guess that's something to put on my "to do" list next time I need to use the charger.:C
