Recharge Voltage

[BlackBart]

Hello,

I was wondering if anyone might know what voltage batteries need to be recharged at when under load. I know that it should be done by about 75% of charge, and that corresponds to about 12.4V when the batteries are at rest. Is there a corresponding voltage under load? Does the amount of load matter?

If someone has any info on this it would be great.

[Rick]

Your information is incorrect. Batteries can be recharged from any level of discharge. When being recharged current flows into the battery only and, by definition, they are not under load. The battery is either under load or it is being recharged (even at float). There is not another condition.

Any external load receives current from the charge source and not the battery. If an external source is present that is incapable of providing sufficient current to totally carry the load then the battery provides the difference and the battery is still said to be under load, not under charge.

[BlackBart]

Thank you for your reply Rick.

I realize that you can recharge a battery when it is at any level of charge, I guess I was speaking of what is generally a good level to not let batteries go below. I don't think running your batteries dead and then recharging them is the best way.

Maybe I can clarify my question a bit. I have a chart that shows battery charge levels and corresponding voltage levels. It uses a battery at rest which would be another condition I guess. It assumes the battery has not been charged or put under load for at least an hour. I am wondering if there is a similar chart for batteries being put under load, or if it remains the same. So if my voltmeter says 12.3v, what level are my batteries at? I hope this helps.

[Therapy]

My understanding is that a 12V battery is fully charged with a resting voltage of 12.6-12.7 volts. It is dead at 12.2.

[Rick]

The battery textbooks show state of charge versus standing voltage. The problem with using this data is fraught with potential error. To be as accurate as possible the battery must be temperature stable and have had no load or charge applied for 24 hours. Such tabular data is very inaccurate for other conditions such as those cruisers encounter. This is why a "real" battery monitor needs to be installed to determine state of charge or infer state of capacity.

With the application of a load one needs to know the dynamic internal resistance of the battery along with the static values for different loads. The one needs to know the resistance of the load. With these values one can theoretically calculate the state of charge (without being able necessarily to calculate state of capacity, what one REALLY wants to know).

Because you don't know state of capacity of your battery without the inference from a real monitor capable of measuring kilo-watt-hour charge and discharge info what good does it do you to infer, for example, an 80% state of charge existant with a battery that has a 40% state of capacity?

You start to get the picture as to why a simple voltmeter is almost as good as useless yet better than nothing.

[David M]

Has your question been answered?

Standard lead-acid batteries need to be brought up to 13.8 volts and held there for some time to receive a full charge. They also need to be equalized periodically which means running the voltage up higher. I try not to let my lead-acid batteries drop below 12.0 volts.

[btrayfors]

BlackBart,

Let's try going at it another way.

1. Whenever you measure 13.8 volts or more on the terminals of a flooded lead acid "12V" battery, it is under charge or, perhaps, still has a bit of float charge left over from recently removed charging current.

2. Whenever you measure less than this amount, the battery is either resting without load or is under load.

I'm curious as to why you're asking the question. Modern smart chargers adapt their charge to the battery condition they sense.

IMHO, typically, charging voltage is TOO LOW. To get a properly and fully charged flooded battery, you really need upwards of 14.2-14.4 at room temperature or, better, with pulse charging you can routinely sustain 15V or more.

TIME is perhaps the most important component in achieving a full charge. The final 10% or so takes a long time, and is important because if the battery is left in less than a fully charged condition for very long, it begins to sulfate. Sulfation (the formation of PbSO4 crystals on the plates) will in time reduce the capacity of the battery to store and to deliver it's designed energy level. As Rick suggested, you won't be able to monitor capacity with a volt meter. You can have a seemingly fully charged battery showing 12.6-12.7 VDC at rest which has a badly depleted capacity -- due to sulfation and/or other factors -- and which will very quickly go dead when a significant load is applied.

Bill