How long to recharge RV battery while driving?

A hydrometer is in my opinion the best way to check state of charge on a battery. Take a reading when your batteries are low before you drive to the next spot, and then take another once you arrive. This will tell you how much of a charge you really got during the time you towed.

I suspect you are probably only getting to the neighborhood of 70% which is a bulk charge. I would also guess the voltage drop in the charge line is too great to get a productive absorbtion charge. Absorbtion is the stage that gets you to about 95% capacity.

But as I stated, a hydrometer check is really the only tool you can bring on the road that will tell you where your batteries are really at. Google handy bobs solar charging puzzle for more info than you ever wanted about RV battery charging.

And 'amp-hour counter' and 'load testers' are two different instruments.

A 'load tester' is what the guys at Autozone use to test the condition of your battery. It puts a very heavy load on the battery of 30 to 40 amps and then measures the voltage while under load.

pianotuna wrote:
Hi JWR and Harvey,

The problem, with just one exception that I'm aware of, is that the "load testers" or "amp-hour counters" need to first be set to the existing capacity of the battery bank.

Except capacity is affected by (not in any particular order) age, design, temperature, Puekerts Law, and several other factors. So it is a moving target.

So the amp-hour counter only gives a (sometimes bad) guess of how much capacity remains. The very popular trimetric doesn't even bother to do a Peukert adjustment.

The only meter that gets more accurate with use is the Smart Gauge.

At the moment, the 556 amp-hour battery bank is charging at 7 amps. One-half of one percent would be 2.78, so my battery bank is not quite full yet. Voltage was showing as 12.9 before I plugged in.

I was experimenting to see if the solar could return the battery bank to full with no outside assistance while I was running my computer constantly, and a heating pad for 3 hours. It almost succeeded.

JWRoberts wrote:
Again, only load testers can answer that question.

Hi JWR and Harvey,

The problem, with just one exception that I'm aware of, is that the "load testers" or "amp-hour counters" need to first be set to the existing capacity of the battery bank.

Except capacity is affected by (not in any particular order) age, design, temperature, Puekerts Law, and several other factors. So it is a moving target.

So the amp-hour counter only gives a (sometimes bad) guess of how much capacity remains. The very popular trimetric doesn't even bother to do a Peukert adjustment.

The only meter that gets more accurate with use is the Smart Gauge.

At the moment, the 556 amp-hour battery bank is charging at 7 amps. One-half of one percent would be 2.78, so my battery bank is not quite full yet. Voltage was showing as 12.9 before I plugged in.

I was experimenting to see if the solar could return the battery bank to full with no outside assistance while I was running my computer constantly, and a heating pad for 3 hours. It almost succeeded.

JWRoberts wrote:
Again, only load testers can answer that question.

Harvey51 wrote:
So if you start charging in the morning with the engine, solar panel or generator, you cannot estimate the level of charge that day using a voltage measurement.

Using an ammeter along with the voltmeter will be at least as accurate as a monitor.

Full charge is 1/2% of capacity in amps when held at absorption voltage.

I lost a good pair of batteries over failing to understand this voltage measurement business, so I will add my $200 worth of learning to this matter. After charging to any extent the battery will measure well over the nominal 12.6 volts for hours. Maybe after no charging for 8 hours the surface charge will be sufficiently absorbed to get an estimate of the state of charge from the rule of thumb for lead acid batteries that 12.0 volts is half charge and as low as you can go before damage to the battery occurs. 12.6 or 12.7 is a full charge. So if you start charging in the morning with the engine, solar panel or generator, you cannot estimate the level of charge that day using a voltage measurement.

The only practical method of getting a good reading on charge level any time you want it is to install a battery monitor. These little computers measure the current flow in and out continuously so they can calculate the level of charge and display it in per cent or number of amp-hours of charge. Price range from $30 to $300. I have one of the $30 ones and I find it excellent.
Battery monitor thread

pianotuna wrote:
I use a volt meter but agree that it takes an extremely long time to recharge. I'm in a class C and have beefed up the charging path, as well as doubling it.

Part of the problem is the engine controllers (ecu) "sees" the starter battery and reduces charging based on the voltage of it. There are ways around this but they require lots of effort as well as modifying the OEM alternator.

I've gone solar instead.

A voltmeter can only measure the surface charge of a battery and is NO indicator of the state of charge. If the voltage is less than 12.6V then the battery is discharged but the voltmeter will not tell you how far below a full charge. And a reading of 12.6V doesn't always indicate a fully charged battery. Again, only load testers can answer that question.

korbe wrote:
Well, I have been using my volt meter and measuring the readings in volts for a lot of years. Doesn't the volt reading measure state of charge? I must be missing something.

You're not missing anything. Using a voltmeter is just fine for assessing the general SOC of your battery provided you measure with the battery's open circuit voltage and take care to avoid measuring surface charge. Those with a significant investment in multiple batteries and a costly solar system will of course want the means to more accurately track what their DC system is doing but for the vast majority of us who just want a general idea using a voltmeter is fine.

Case in point - some years ago we we dry camped in Yellowstone for 4 nts during July and our one G27 battery handled the task just fine. On the morning we were leaving I measured the battery's open circuit voltage with a DVM and saw ~ 11.9 volts ... we towed all that day and intentionally camped that night at a KOA because I knew we'd have 120 vac shore power available for fully recharging the battery. Upon our arrival around suppertime and before plugging in I bled off battery surface charge with a light bulb, then measured the battery's open circuit voltage at ~ 12.6 volts so obviously the charge line from our Explorer did accomplish a decent battery recharge over the course of 8 hrs of driving that day. The next morning we left the KOA and headed to RMNP where we successfully dry camped for several more days with no issues at all.

2oldman wrote:

korbe wrote:
Well, I have been using my volt meter and measuring the readings in volts for a lot of years. Doesn't the volt reading measure state of charge? I must be missing something.

It's good enough for me.. as long as that voltage stays there when the charger is removed.

You can read about SOC hereSmart Gauge and hereBattery FAQ. Bottom line, most of us use voltage readings after the battery has rested 4 to 6 hours under NO LOAD conditions. It is a ball park reading.S.G. readings that are temperature corrected under the same NO LOAD conditions will get you into the infield of the ball park.
Another link on SOCBattery University

I use a volt meter but agree that it takes an extremely long time to recharge. I'm in a class C and have beefed up the charging path, as well as doubling it.

Part of the problem is the engine controllers (ecu) "sees" the starter battery and reduces charging based on the voltage of it. There are ways around this but they require lots of effort as well as modifying the OEM alternator.

I've gone solar instead.

korbe wrote:
Well, I have been using my volt meter and measuring the readings in volts for a lot of years. Doesn't the volt reading measure state of charge? I must be missing something.

It's good enough for me.. as long as that voltage stays there when the charger is removed.