Some battery questions

[GallenH]

Ok. These are going to seem obvious to most of you but they are things that I have wondered about for a year or so. My situation is a single 12v AGM cabin battery that I supplement with a 100w folding solar.

1. Don't go below 50% on the battery to not damage and extend life. How do you determine 50%?

2. What do you do if your battery is hitting 50% in, say, the middle of the night? Shut everything down?

3. My solar controller monitors battery voltage but it's only active when I connect up solar. Should I have a volt meter? Suggestions?

4. When I put the RV in storage, I disconnect the ground to the cabin battery. Then as I'm prepping for a trip, I plug the RV into shore power at my home. This should charge the battery, right? I have a spare battery tender. Would it be better to use that to get the battery up to full charge or use the convertor/charger that's built into the van?

Thanks!

Thanks!

[booster]

Originally Posted by GallenH

Ok. These are going to seem obvious to most of you but they are things that I have wondered about for a year or so. My situation is a single 12v AGM cabin battery that I supplement with a 100w folding solar.

1. Don't go below 50% on the battery to not damage and extend life. How do you determine 50%?

2. What do you do if your battery is hitting 50% in, say, the middle of the night? Shut everything down?

3. My solar controller monitors battery voltage but it's only active when I connect up solar. Should I have a volt meter? Suggestions?

4. When I put the RV in storage, I disconnect the ground to the cabin battery. Then as I'm prepping for a trip, I plug the RV into shore power at my home. This should charge the battery, right? I have a spare battery tender. Would it be better to use that to get the battery up to full charge or use the convertor/charger that's built into the van?

Thanks!

Thanks!

You need to get a battery monitor like a Trimetric if you want to actually know for sure what your state of charge is all the time. Well worth the cost and effort to install IMO.


You may also want to search here and find some the discussions about the 50% rule. I think you will find that going under 50% once in a while would not appreciably change the life of you battery. You are much, much, more likely to shorten you battery life because of poor charging that gives either over or under charging. A battery monitor will very quickly show you how good, or bad, your charging systems are working to take good care of your battery.

[markopolo]

Booster has been doing his best over the years to get us to think of battery output in terms of ENERGY and to stop worrying about CYCLES.

Take a look at the scary cycle life chart on this 6V AGM battery data sheet: https://www.amstron.com/content/AP-G...tion_Sheet.pdf

read between the lines.JPG

If you read between the lines you see that the energy that you get out of the battery at the different levels of discharge isn't scary at all!

When I read between the lines I see:

2 x GC2 210Ah 6V batteries (210Ah @ 12V)
500 cycles at 100% DOD = 12v X 210Ah (100% DOD) X 500 = 1,260 kWh total output until replacement
675 cycles at 80% DOD = 12v X 168Ah (80% DOD) X 675 = 1,360 kWh total output until replacement
1100 cycles at 50% DOD = 12v X 105Ah (50% DOD) X 1100 = 1,386 kWh total output until replacement

The difference between 50% and 80% DOD is so minor that it is not worth worrying about. Most people fail to make the connection that deeper discharges mean that you used more energy. Energy output is what you purchase a battery for.

If you have 10 candy bars (energy supply) and eat 1 per day (Depth of Discharge) you'll have candy bars (some energy) for 10 days. If you eat 2 candy bars per day (greater DOD, more energy consumed) your candy bar supply will only last 5 days.

Boosters suggestion of adding a monitor to know the state of charge is the way to go. It is really the only way to know that you've fully charged the battery. Getting the battery full is critical to lead acid battery life.

You can try to manage the battery by monitoring voltage. It is no where near as effective as using a SOC monitor. To check voltage you have to first turn off any loads like lights, furnace or fan etc.

Phillips State of Charge chart flooded and agm.JPG
interstate chart.JPG
Never go below 12.0V resting voltage could be one of your basic rules.

[markopolo]

Originally Posted by GallenH

...................

4. When I put the RV in storage, I disconnect the ground to the cabin battery. Then as I'm prepping for a trip, I plug the RV into shore power at my home. This should charge the battery, right? I have a spare battery tender. Would it be better to use that to get the battery up to full charge or use the convertor/charger that's built into the van? ............................

It's unlikely that the converter (unless newer 3-stage) or the trickle charger gets the battery full. The same applies to alternator charging particularly if a diode type isolator is in use.

It's quite possible that you are continually in partial state of charge cycling that results in shortened battery life.

[booster]

All that Marko said is right on, especially on the very high probability that you are essentially never getting your battery fully charged.

[GallenH]

Thanks booster and markopolo.

So I've seen voltage monitors for RV applications that range from $18 to $300. Is the difference quality/accuracy or the range of things that the monitor does?

Also, markopolo, you mention that it's possible to be in a constant state of charging with the onboard or trickle chargers. What is the solution?

Thanks.

[booster]

Originally Posted by GallenH

Thanks booster and markopolo.

So I've seen voltage monitors for RV applications that range from $18 to $300. Is the difference quality/accuracy or the range of things that the monitor does?

Also, markopolo, you mention that it's possible to be in a constant state of charging with the onboard or trickle chargers. What is the solution?

Thanks.

You don't want a voltage only gauge for battery monitoring. You need a true battery monitor that keeps track of the power you use, and the power you put back into the battery. This is one, but shop around.


https://www.amazon.com/TriMetric-TM-...monitor+system


You would need better charging equipment to do a better job of getting the battery fully charged.

[markopolo]

Originally Posted by GallenH

..................Also, markopolo, you mention that it's possible to be in a constant state of charging with the onboard or trickle chargers. What is the solution? ..................

I was trying to explain that you're likely always at a partial state of charge and cycling the battery in that state. You might get to 90% when plugging in before a trip. Then you use the battery and it gets down to 40%. Then your solar charges it back up to 80% and that night you take it down to 50% SOC. You might never be getting that battery full. Not getting the battery back to 100% SOC allows more lead sulfate to form on the battery plates. More lead sulfate = earlier loss of capacity & earlier battery failure.

https://www.practical-sailor.com/iss...s_11691-1.html

A partial state of charge cycle test is testing in which the battery is discharged and partially recharged but does not return to 100 percent before being cycled again. This mimics what happens when we’re at anchor or when we store our boat on a mooring or in a marina with no electricity. In these cases, partial state of charge operation is a fact of life—a fact of life that can be murder to batteries.

Fortunately with RV's we're better able to care for the batteries because we have more opportunities to charge the batteries than boaters get.

[cruising7388]

Originally Posted by markopolo

It's unlikely that the converter (unless newer 3-stage) or the trickle charger gets the battery full. The same applies to alternator charging particularly if a diode type isolator is in use.

I think modern vehicle computers examine battery voltage and adjust alternator output voltage to compensate for diode or other losses.

[booster]

Originally Posted by cruising7388

I think modern vehicle computers examine battery voltage and adjust alternator output voltage to compensate for diode or other losses.

They will only do that if you move the voltage sensing wire to the coach batteries instead of the starting battery. There is also a risk to it in that the voltage the van and starting battery can see will be high enough to cause in some cases. Think and alternator stabilizing at 14.5 volts, plus add on a volt for isolator, and you would be putting 15.5 volts on the van electronics.



The moving the sense wire was pretty common a decade ago, a hangover from when alternators usually ran at about 13.8v in the further past.

[markopolo]

Originally Posted by cruising7388

I think modern vehicle computers examine battery voltage and adjust alternator output voltage to compensate for diode or other losses.

I don't see how the alternator senses voltage beyond its first (and possibly only) connection point to a battery but would be happy to read any links you can provide.

(typed while Booster was posting)

[avanti]

Modern vehicles go far beyond simple voltage sensing. They modulate the alternator to improve acceleration. They deliberately keep the charging battery less than full so that they can use the space to recover energy while coasting, etc. etc. There are sensors all over the place to accomplish this kind of thing.

The days of hacking into these systems to charge house batteries is passing quickly. This is a main reason why second engine alternators are gaining favor.

[markopolo]

How does a computer controlled modern vehicle alternator handle continuous loads like lights at night?

I wonder if the modulation is opportunistic meaning the priority would be to respond to loads if they're there. Would it see an auxiliary battery draw as a priority and maintain output?

[avanti]

Originally Posted by markopolo

How does a computer controlled modern vehicle alternator handle continuous loads like lights at night?

I wonder if the modulation is opportunistic meaning the priority would be to respond to loads if they're there. Would it see an auxiliary battery draw as a priority?

The exact algorithms don't appear to be generally known, and no doubt vary by manufacturer. Note, however, that in such vehicles, all significant loads are controlled via the CANbus, and so the ECM is aware of them. My guess is that they are compensated for in that way, rather than by direct measurement of the actual load (which would be manifest as voltage drop, and thus would behave more like the older systems). The Sprinter I4 engine has such a system, and there is a set of sensors (probably including a shunt) mounted directly on one of the battery terminals, so it is directly watching the current into and out of the battery. How it "sees" a house battery would be very dependent on which side of this sensor you took the current.

MB has for a number of years required upfitters to limit the take-off current to a rather low value. No doubt this was in anticipation of these new systems.

[booster]

I have no idea if it is correct or not, but a tech I talked to said that they monitor the vehicle itself for the serious control of battery charging, so the basically try to only turn on the battery charging when there is a deceleration happening. It wasn't clear if they actually switched the alternator off the rest of the time and ran off the battery, or if they just held shut the battery itself off, or is they just reduced the voltage enough to stop the charging. They have monitored almost all the high power user circuits for years, mostly to kick up idle speed in the past, so I would certainly think they still do that. I would think that if anyone has a factory service manual it would likely explain the theory, if not the details, as that is the kind of thing they did in the past, at least on US company vehicles. I wouldn't expect MB to say anything except take it to the dealer and be sure to bring lots of money

[ndavidking]

Based on all of this excellent advice I have a related question about batteries and storage. Since I swapped my Class A for a far more practical Class B I can now park it in my driveway. Would it be better for the batteries to just keep it plugged in to shore/house power whenever it is parked or let them naturfally discharge over time and then periodically plug it in?

[booster]

Originally Posted by ndavidking

Based on all of this excellent advice I have a related question about batteries and storage. Since I swapped my Class A for a far more practical Class B I can now park it in my driveway. Would it be better for the batteries to just keep it plugged in to shore/house power whenever it is parked or let them naturfally discharge over time and then periodically plug it in?

That is an often discussed question, and I don't know that we ever got a good answer to it. The manufacturers of the batteries are squishy on an answer usually saying stuff like small discharges and recharging are fine but have to have the recharge fairly soon after the discharge, etc, but with no real guidelines. For us, that would eliminate just unplugging the van and shutting everything off as it will barely discharge over time, and even with a small load could be many weeks before recharging. We do leave it plugged in all the time now with the Lifeline AGM batteries, but if I am going to be doing stuff in the garage either to the van or other stuff, I sometimes will just turn on the inverter with a small heater running to discharge maybe 10% and then recharge them and let it go back to float. Be aware that a very small discharge like that may keep some chargers from doing a full charge as they look at starting voltage, so you might have to a bit deeper.

[ndavidking]

Thanks. It is interesting that what must be a common question for RVers has never gotten a good and final answer especially from battery manufacturers. Surely that would not be because they most of all want us to keep buying new batteries...??? I think in the meantime, your approach is a good one and will adopt that. But if some different wisdom appears on the scene I'll bet it would be graciously received by many of us.

[RossWilliams]

"When I read between the lines I see:"

I think you read wrong. A battery cycle is always a 100% discharge, so 5 80% discharges is the same as 8 50% discharges, both are 4 battery cycles. You get more total power if you get more cycles.

Also, there is nothing magic about 50%, the number of cycles increases even more if you keep the discharge lower than 50%. You are probably better off with a 10% than 5%, but anything more than that you are shortening the battery life and its a tradeoff for usefulness.

That also means the larger your battery array, the longer it is likely to last. When you are talking about heavy loads like air conditioning, there is another advantage to a larger array. Battery capacity does not decrease at a linear rate. A heavy load will reduce the remaining battery capacity faster than the same wattage spread over a longer time. A 1000 watts for an hour will draw the battery down further than 100 watts for 10 hours. And, like the question of battery cycles, the amount of that difference is tied to the battery array's capacity. Drawing down by 80% over a couple hours increases the effect more than drawing down only 50% even if the watts used is the same.

Battery management is far different than holding tanks. There is a lot more to it than wattage in, wattage in storage and wattage out. But more is almost always better.

[booster]

Originally Posted by RossWilliams

"When I read between the lines I see:"

I think you read wrong. A battery cycle is always a 100% discharge, so 5 80% discharges is the same as 8 50% discharges, both are 4 battery cycles. You get more total power if you get more cycles.

Also, there is nothing magic about 50%, the number of cycles increases even more if you keep the discharge lower than 50%. You are probably better off with a 10% than 5%, but anything more than that you are shortening the battery life and its a tradeoff for usefulness.

That also means the larger your battery array, the longer it is likely to last. When you are talking about heavy loads like air conditioning, there is another advantage to a larger array. Battery capacity does not decrease at a linear rate. A heavy load will reduce the remaining battery capacity faster than the same wattage spread over a longer time. A 1000 watts for an hour will draw the battery down further than 100 watts for 10 hours. And, like the question of battery cycles, the amount of that difference is tied to the battery array's capacity. Drawing down by 80% over a couple hours increases the effect more than drawing down only 50% even if the watts used is the same.

Battery management is far different than holding tanks. There is a lot more to it than wattage in, wattage in storage and wattage out. But more is almost always better.

"When I read between the lines I see:" Was Markopolo's post, but I will comment, as I don't think he is incorrect in his evaluation


Your definition is exactly what we were discussing earlier about if a "cycle" is a discharge/recharge event of any depth, or a cumulative of discharge/recharges that total 100% as was mentioned as how Apple lists it. RV batteries, based on the charts they give, look to nearly all use the former definition of any depth discharge and recharge to full is one cycle. As mentioned, neither is wrong as long as it is stated which way is being used. In your example, you will get the same amount of total watt-hours out the two discharge scenarios, but the 80% discharge will reduce the total lifetime watt-hours that the battery will store and release before worn out, by about 15%, per the battery manufacturer's life charts based on DOD and recharge cycles (defined as any depth)


In the second example, you referring to Peukert compensation, and this also appears to be a bit of a definition and consistency thing. First is the definition of a discharged battery, which for the these types of batteries is when the battery gets to 10.5v. What is not defined is when that 10.5v occurs, so it can be at any, or no, load size. If you have a big load, the load pulls the battery voltage down due to internal resistance, so you hit the 10.5v at lower used watt-hours than you would at a smaller load with less voltage drop. Where this get interesting is if you go to 10.5v with a bit load, and then let the battery rest for a few hours, it will recover voltage as we have all seen in practice in our RVs. If you then put the small load on with the lower voltage drop, it will then run some amount of time longer before it again hits 10.5v. What is interesting is that the total watt-hours the battery will give up is very close to identical in both examples, which makes total sense because otherwise you would have mystery energy disappearing. The bottom line of this is that the high discharge rate does not use more of the energy stored in the battery, than a lower rate would, for the same amount of watt-hours removed from the battery. All the high rate does is get you to the definition of a discharged battery earlier because of internal voltage drop due to high amperage. The energy difference compared to a slow discharge is still in the battery, and can be used if at a smaller load. And yes, I have tested this twice on our system to make sure it happens.


A perfect demonstration of how this works is something that nearly everyone with a fairly large inverter has experienced. You are running the inverter at big load (microwave?) and it trips the low voltage cutout and shuts off. If you have the cutout set at 10.5v, by definition the battery is empty, so you should not have any power left to extract from the battery. But, after the big load is off, the battery is reading 12.0v or some other non dead voltage, and is capable of running stuff other than the microwave for a long time on the power that is still in the battery and not lost to the high amps. Depending on bank size and battery type, you might have 30% or more capacity left.



All of this comes into play for the calculations of benefit/detriment of big vs small battery banks. Cost, weight, use patterns all come to play, and there is no one size fits all answer as sometimes small is better, sometimes big is better, in terms of the balance of all factors.


This is all very fascinating stuff and not nearly as simple as so many of the "rules" would have us believe.