Underhood generator testing

[booster]

I thought you can charge AGM batteries at 20% of capacity. So how are you using the 180 amps? It does make a good case for lithium battery bank. That is a lot of amps in a short period.

You can charge AGM at 20%C, it is actually what they recommend as a minimum for deep discharge use. At about 40%C, they will start to get pretty hot, so I would recommend that as a high end except for very short charges like to get back a days capacity in a hurry.


For us 180 amps is 41%C for our 440ah bank and AGM Lifelines.

Wet cells do start to limit off at about 20%C due to excessive gassing.

 

[prfleming]

https://www.powerstream.com/SLA-fast-charge.htm

Here is an interesting article on high charge rates. It states that no oxygen is generated during charging, i.e. bulk charge, only during overcharging.

In testing my 400 AH bank, I have discharged to approx 50%, then measured the bulk charge from underhood generator at 200 amps, which tapered off as batteries reached full charge. After this test, batteries were cool to the touch.

 

[booster]

https://www.powerstream.com/SLA-fast-charge.htm

Here is an interesting article on high charge rates. It states that no oxygen is generated during charging, i.e. bulk charge, only during overcharging.

In testing my 400 AH bank, I have discharged to approx 50%, then measured the bulk charge from underhood generator at 200 amps, which tapered off as batteries reached full charge. After this test, batteries were cool to the touch.

The Powerstream articles scare me somewhat as they are contrary to what the actual manufacturers of the batteries recommend in a lot of things. High rate charging is not the big deal, as the batteries will accept it, it is the temp rise from the high rate that is what you have to look out for IMO, as long as you are under 1C.

What I can tell you for certain is that at 40%C on our Lifelines, they will be at about 10-15* above ambient if brought up from 20% SOC. That would be 110*F on a hot day and about as hot as you really want to be. Temp compensation will really knock the rate down, though is you have it.

At 50% state of charge, you are only going to see 200 amps for a very short time, as you will start to taper at about 70% SOC, so barely over 1/2 hour of charging, and the batteries will cool as the rate tapers to full, which will take upwards of 8 hours more time. We can charge at 285 amps (65%C) for about 20 minutes without getting very hot so time is very important in high rate temp rise, as is battery location. Ours are underneath the van, so we can get about 10-15* of temp increase from the heat of driving, so we are pretty careful.

 

[pearll]

I thought AGM batteries will charge in about 5 hours. Charging at 20% of capacity, it will charge about 30% in 1.5 hours. The remaining 20% will take quite a while, for a total of about 5 hours. Why install a bigger alternator if the battery bank is not going to get to a full charge any faster?

 

[booster]

I thought AGM batteries will charge in about 5 hours. Charging at 20% of capacity, it will charge about 30% in 1.5 hours. The remaining 20% will take quite a while, for a total of about 5 hours. Why install a bigger alternator if the battery bank is not going to get to a full charge any faster?

There are a couple of advantages of the big alternator or shore charger with AGM batteries.

First would be that they will accept the high rates from the 20% SOC to about 70%, so you can recover several days worth of use in a very short time, you just can't get all the way full at the high rate. Most manufacturers say that if you do get totally full every 7-10 charge cycles, you will not have battery capacity walk down due to not getting fully charged.

The second would be that if you are on a driving day, and you have solar, you can get recovered to the 70% level very quickly, leaving enough time for the lower rate solar to finish a complete charge, even after you have stopped driving, thus fullfilling the fully charged every 7-10 cycles requirement for good battery life.

Many people think that their batteries are full in "X" number of hours, but unless you are checking the acceptance amps and absorption voltage, you really don't know if they are full or not. Our 440ah of Lifelines, with a 100 amp shore charger, will take about 10 hours to go from 20% to totally full.

 

[avanti]

With AGM, you just have to be prepared to not worry about the last 10% (or rely on solar for it). This is fine as long as you go to 100% occasionally for battery health reasons.

 

[pearll]

I did not realize it took that long or that realistically you have less than 50% of usable capacity. Along with the claimed longer life, it makes the idea of switching to lithium batteries more appealing.

 

[avanti]

I did not realize it took that long or that realistically you have less than 50% of usable capacity. Along with the claimed longer life, it makes the idea of switching to lithium batteries more appealing.

Well, if I have this straight, you don't really have less than 50%. My understanding is that discharging from 90% to 40% is not materially worse than discharging from 100% to 50%. Someone correct me if I am wrong in this.

 

[pearll]

That is true but @50% discharge, you have 500 cycles and @80% discharge, you half 1/2 that with 250 cycles. So always discharging to 60% would cut down on life of battery, I think.

Some people seem to see their AGM batteries as a kind of disposable item, use them pretty harshly and just change them more often than others. Seems kind of expensive but maybe that is just part of their budget and it works.

 

[BBQ-ClassB]

I did not realize it took that long or that realistically you have less than 50% of usable capacity. Along with the claimed longer life, it makes the idea of switching to lithium batteries more appealing.

Lithium has many desirable features.

Lithium is an alternative, but not a substitue.

Everything has a price (trade-offs), and the price (including non-monetary ones) is not cheap.

 

[booster]

That is true but @50% discharge, you have 500 cycles and @80% discharge, you half 1/2 that with 250 cycles. So always discharging to 60% would cut down on life of battery, I think.

Some people seem to see their AGM batteries as a kind of disposable item, use them pretty harshly and just change them more often than others. Seems kind of expensive but maybe that is just part of their budget and it works.

This one comes up all the time here, and there are a couple of very good discussions of it on this forum. I will dig up the links later. Here is a link to one of them, that also links to the original discussion.

http://www.classbforum.com/forums/f23/another-view-of-the-50-agm-discharge-limit-5595.html

What we are talking about in pearll's example is the old "50% rule" that says that if you go under 50% SOC on your batteries they will not last long. While the number of cycles is double at 50% SOC compared to going to 20% SOC, that doesn't mean the batteries last twice as long because if you go to 50% instead of all the way to 20%, you have to charge more often. We have shown on this forum that the total number of amp hours stored and then used, which is what really counts in battery use, is closer to only 10% different between discharging to 50% or down to 20% SOC. Add to that the fact that the discharge depths "average" out, and that gets rid of the theory that even a couple of discharges to 20% will permanently damage your batteries. Two discharges to 20% SOC plus two smaller discharges to 80% SOC come out to be essentially the same in regards to life, and energy stored and used, as 4 discharges to 50%.

We have found that this is very hard for a lot of people to believe, as the old "rules" have been around for a long time, but all real data we have seen would indicate it to be correct.

In real life, this can have some very large influences. For a long time, because of the 50% rule, we were told an AGM bank had to be double what you actually used because you could "never go under 50%". Going to 20% gives you 60% more capacity than going to 50% SOC, so you have fewer batteries, less cost to buy, and less weight to haul around all the time, and even in the very worst case of always discharging to 20% SOC (which hardly every happens in the real world, as we all get varied discharge depths) you lose only 10% of battery life.

 

[booster]

Well, if I have this straight, you don't really have less than 50%. My understanding is that discharging from 90% to 40% is not materially worse than discharging from 100% to 50%. Someone correct me if I am wrong in this.

I think you have a combination of things in what you are saying, some of which would be true IMO, and some that are somewhat true, again IMO.

The thing that makes some of the stuff not quite true is that in one case you are getting to a full charge on the batteries, and in the other you are not, which will cause battery capacity "walkdown", if you don't get a a periodic full recharge.

If you do get the periodic full recharge, every 7 cycles or so, then the difference of 90-40 or 100-50 percent of charge will have a very small effect on battery life.

On our system, what we have found is that we can get back to 70-80% very quickly with big alternators or the 100 amp shore charger, usually about 1-2 hours, and then it takes another 6-8 hours to top off the batteries to totally full. By the time we get to 80% full the solar is usually putting out enough amps to take over so we don't need to drive at that point as the solar will finish the charge.

Of the 6-8 hours to top off, probably close to 2 hours will be needed to get from 80-90%, so we normally wouldn't consider running the engine to get that 10% of capacity as it is a waste of time and gas.

If we are not in a position to get totally full, we will run between 20 and 80% SOC without hesitation, and just make sure we get a 100% full charge once a week, either by driving longer and topping with solar, or by getting shore power for a night.

I will also again point out that most people don't really know when their batteries are really full unless they have a properly setup battery monitor installed. Nearly every one of them that I have personally seen, do not have the correct fully charged parameters set in them, and are saying the batteries are full before they really are. For the 440ah of Lifelines we have, we need to get to 2 amps of charging current at absorption voltage to be full, which is really a small amount.

 

[wincrasher]

This one comes up all the time here, and there are a couple of very good discussions of it on this forum. I will dig up the links later. Here is a link to one of them, that also links to the original discussion.

http://www.classbforum.com/forums/f23/another-view-of-the-50-agm-discharge-limit-5595.html

What we are talking about in pearll's example is the old "50% rule" that says that if you go under 50% SOC on your batteries they will not last long. While the number of cycles is double at 50% SOC compared to going to 20% SOC, that doesn't mean the batteries last twice as long because if you go to 50% instead of all the way to 20%, you have to charge more often. We have shown on this forum that the total number of amp hours stored and then used, which is what really counts in battery use, is closer to only 10% different between discharging to 50% or down to 20% SOC. Add to that the fact that the discharge depths "average" out, and that gets rid of the theory that even a couple of discharges to 20% will permanently damage your batteries. Two discharges to 20% SOC plus two smaller discharges to 80% SOC come out to be essentially the same in regards to life, and energy stored and used, as 4 discharges to 50%.

We have found that this is very hard for a lot of people to believe, as the old "rules" have been around for a long time, but all real data we have seen would indicate it to be correct.

In real life, this can have some very large influences. For a long time, because of the 50% rule, we were told an AGM bank had to be double what you actually used because you could "never go under 50%". Going to 20% gives you 60% more capacity than going to 50% SOC, so you have fewer batteries, less cost to buy, and less weight to haul around all the time, and even in the very worst case of always discharging to 20% SOC (which hardly every happens in the real world, as we all get varied discharge depths) you lose only 10% of battery life.

No, you only loose 10-12% of the total energy out of the battery. You certainly loose 1/2 or more of the battery life, which is cycles. So you have to consider if your goal is total daily energy output, or the total longevity is what is important. There is no free lunch - one comes at the expense of the other.

 

[booster]

No, you only loose 10-12% of the total energy out of the battery. You certainly loose 1/2 or more of the battery life, which is cycles. So you have to consider if your goal is total daily energy output, or the total longevity is what is important. There is no free lunch - one comes at the expense of the other.

Number of cycles is only important if you have a fixed schedule that has a predetermined amount of discharge and a recharge for that discharge every time it happens. but that is not how it happens in RVs.

Amount of energy in and out of the battery is what counts most in RVs, IMO, for most of us.

If you take your batteries to 20% SOC in two days, and recharge once, or you take them to 60% SOC every day and charge twice, not a real be big difference in total energy used (like none), but you have double the recharge cycles on the shallow discharge, so break even on that too, if you only get 1/2 the cycles at the 20% point. Plus, if you only go shallow all the time, you don't have the option of using less batteries at less cost and less weight.

Put another way, how much difference in cost, and how much difference in run time of your lawn mower, would there be between buying a years worth of the gas for it in 5 gallon cans or 2.5 gallon cans?

If I can have half as many batteries to get the same usable capacity, with only a 10% penalty in cost, I will take it every time. Heck, probably save that much on gas from not dragging so much weight around.

I think that this idea the recharge cycles is the only thing that matters in battery life and use is why the 50% rule is so stubbornly defended by lots of people.

 

[avanti]

Although I am by no means an expert, I think I am going with Booster's interpretation on this one. Before you could claim that "cycles are what matters", you would have to define a "cycle". Would 30% down followed by a full recharge count? How about 10%? or 1%? How about going from 60% to 40%?...

I have seen zero evidence that there is any kind of threshold effect at the proverbial 50%. I doubt that there is one anywhere--chemistry tends not to work that way. The evidence seems to indicate that it is total amps out/in that matter. Now it is certainly possible that it is not linear--i.e., perhaps going from 40% to 30% is more harmful than going from 90% to 80%, but as far as I can see the data are lacking.

 

[markopolo-ClassB]

In addition to capacity degradation from cyclic use there's also unavoidable calendar aging to consider. That fact would support more fully utilizing an adequately sized (not oversized) battery bank. In other words - don't worry about adherence to the 50% DOD guideline but still practice good battery management.

The batteries won't last forever because the negative effects of the chemical process can't be avoided. The clock is ticking so it would seem to be better (in a typical RV type use context) to use the output rather than let it slowly waste away.