Discharging AGM batteries beyond 50% - could it be a smarter choice?

[adam88]

Let's have a discussion about discharging batteries please. First, let me post some data about Lifeline AGM batteries. According to this page I found from Norhaven http://www.nordhavn.com/resources/tech/battery_death.php (they package lifeline AGM batteries into large cell packs), the average lifespan for Lifeline AGM batteries is:
.
50% average discharge = 1000 cycles
70% average discharge = 650 cycles
95% average discharge = 400 cycles
.
Now let's say I want a camper that has a useable 255 amp hour battery bank. Traditional logic (and many folks here on expo) would say that I should buy TWO 255ah 12v batteries, or double the capacity I want to use, and never exceed 50% discharge. In this case, total cost for the batteries would be $1300. Total weight would be 312 pounds (each battery weighs 156#). They would last me for 1000 cycles, or 500 cycles PER battery.
.
Now let's compare that to just one battery that is basically fully discharged to 95%. It would get roughly 400 cycles, which is 20% less cycles than the double capacity, but you save 50% of the weight. That's 20% less capacity, 50% less weight. Seems like a reasonable trade-off to me.
.
Let's go a little deeper and propose that the dual battery combo may actually cost more in the long run. Why? Well, 156# extra over the course of tens of thousands of road miles = worse gas mileage. Even if that 156# only reduces gas mileage by, let's say, 0.2mpg, it would still have a significant impact on fuel costs over 5 years. The amount saved on fuel could easily pay for the 20% difference in cost.
.
The vehicle would also handle better with less weight, etc, etc. Basically the point of this thread is to point out that discharging a battery beyond 50% is not the deadly sin it is made out to be. In fact, it may actually be an economically better choice, and reduce weight in a camper.
.
AGREE? DISAGREE? DISCUSS!!!!!

 

[1leglance]

Switch to Lithium
That is what I am watching, and when my 2 AGM's go out more than likely I will switch battery type.
Otherwise I can see your point, but I am not smart enough to say much else

 

[adam88]

Switch to Lithium
That is what I am watching, and when my 2 AGM's go out more than likely I will switch battery type.
Otherwise I can see your point, but I am not smart enough to say much else

I'm not smart enough either However, some things I can see being discussed include:
.
- With double the capacity, the batteries will discharge slower at the same amp draw. If you cut the battery bank in half, then you need to cut the discharge rate in half too or you'll end up with a faster discharge. In other words, running a 20 amp draw on 1 battery vs 20 amps on 2 batteries, the two batteries would outlast the one battery by more than double.
.
- Having "cushion" built into the battery bank with the double battery option. E.g., aim to never drop below 50%, but that 50% is there if you need it in an emergency or for some other reason.
.
I suspect that I will still end up going with the traditional logic and doubling capacity, but maybe not.
.
As for lithium batteries, I am really interested in them. Last I checked though, the cost was about 2x as much as the Lifeline batteries per kW. How are prices on them?

 

[IdaSHO]

Interesting way to look at it.


But do now forget about the additional charging amps required when you push beyond 50%.

To get even the rate 400 cycles out of that battery bank discharging to 95% you need to recharge completely on a regular basis.


Easy numbers....

200AMP battery discharged 50% in 24hrs uses 100 amps.
So you must supply that bank with 100 amps of charge regularly (ideally every day)

That same battery bank discharged to 95% in 24 hrs uses 190 amps
Now you need 190 amps of charge to maintain that bank, again ideally daily.

In a conservative 6 hours of solar, that's a 17amp vs 32 amp per hour system, or basically a 210 Watt vs 400 watt solar array.


All the while trying to permanently damage the battery bank.
Every time you discharge that deep you run the risk of never being able to bring the bank back, assuming is is a traditional (non Lithiom) bank.

 

[Ducky's Dad]

I don't know about the Lifelines, but I'm pretty sure that with other AGM batteries (such as Odyssey), the battery will gradually lose its ability to take and hold a charge if it is deeply discharged (despite the fact that they are advertised as deep cycle batteries). That means that the capacity you start out with will not be the capacity you have at, say, 200 cycles into the battery's life.

 

[IdaSHO]

That is correct, and is with any traditional (non lithium) battery

Regardless of how far it is discharged, it looses their ability to recharge 100%

That said, the deeper it is discharged the more it looses its ability to hold a charge

 

[DiploStrat]

In my discussions with Lifeline they have stressed:

-- For the greatest number of cycles, discharge less, recharge faster. (I.e. don't let the battery sit around discharged for several days.)
-- Always recharge as completely as possible - this requires higher voltage and longer time - hence my advocacy of solar or shore charger.
-- You CAN discharge more deeply (that is, break the 50% "rule") but if you do so, it is especially important that you get the battery back on charge as fast as possible, for example, the next morning.

Thus a full recharge on a regular basis is more important than the occasional deep discharge.

As always, this is my understanding of what I was told; YMMV.

 

[4x4junkie]

.
- Having "cushion" built into the battery bank with the double battery option. E.g., aim to never drop below 50%, but that 50% is there if you need it in an emergency or for some other reason.

This is my main reason for having more capacity than I use typically... For those few times I do need more capacity, it'll be available for me to use (plus you can never fully predict the weather, a long enough string of cloudy days can allow even a quadruple-sized battery bank to become fully discharged when your solar harvest is reduced accordingly). Additionally, more capacity also helps maintain higher working voltage as well, which most loads will run a little more efficiently when the battery voltage is above 12.1V or so (about what a battery @ 50% SoC would maintain while under a light-moderate load).

 

[scott7022]

While I am happy to see Lithium tossed around on the board nowadays, I think its funny I am now considering going to a Lifeline product. I was all hard set on my build going "lithium" as I need power for a 5k editing studio on board, charge equipment and run all the crap that keeps me alive and getting paid. Each time I solved a difficulty with lithium I encountered another past the sticker shock price. The cost factor is only justifiable to those that are off grid and require the power past just the "cool" factor. The OP has a very valid argument. Lets use the Original Lifeline currently for sale for $651 (regular list suggested is around $1000). The Relion RB150 $1898 and you're going to pull 120amp hours at 80%. For the LifeLine this is 204amp hours at 80% depth of discharge. So to get an apples to apples you need to step up to the Relion RB260 to get 208amp hours at 80% DOD, and that little lady is $3470. The RB260 SAYS it will give you 2000 cycles but... The issue the OP is leaving out past the cost, is primary power pack failure. Ripping the SINGLE Lifeline down 80% is not only going to increase charge delivery times, as already pointed out. One day it is just going to die. You know the old saying in terms of WHEN this will happen. -40, 300 miles from nowhere.
I agree the math makes sense and it isn't just cheaper battery wise. Charging differences between the two systems is exacerbated by mixing technology made for traditional dual battery systems with new Regenerative Braking systems and voltage regulation buried into the computer of the vehicle. Most of the experts in this regard are not on this side of the world, sorry to say. We have some and they are as expensive as the lithium batteries they sell. This will change, just as the battery costs have changed since TechnoMedia first did their install. I was interested way back then. The batteries have come down huge since then cost wise, but simply grabbing a great dual battery unit with a 400amp relay, with a cool glowing square for info and combining isn't going to work. Eventually the BMS system is going to die, in newer vehicles, via spikes or worse. For lithium you need a DC to DC charge unit. My new big dual alternator Dodge tosses out 380 amps. This is past the current handling of most DC to DC units currently available here. Australia has one that will handle it. Dual EnerDrive units, but then we run in other issues as I mentioned at the start.
I originally was going to use lithium as I need about 500amp/hrs with headroom for lithium as they are too expensive to abuse. So that is two of the those little ladies listed above. Add dual Enerdrive units, without support or warranty, at 800 bucks and I am still mixing chemistry, starting and house. It gets complicated and fall back options sharply diminish if something goes south.
For me Lifeline AGMs make more sense. Dual Alternators dual stock front batteries come directly off each engine starting battery, with that cool glowing green product I mentioned and into a magnetic breaker rated for 180 amps and then into dual DC to DC chargers rated the same and two gauge back to the three Lifelines. Yeah it is heavy but the murphy factor is zero, well as zero as Mr. Murphy gets.
I think the OP's idea is workable by adding a little group 27 and a drain intelligent protector that stops the group 27 from being pulled down and only connects when the "drained" unit is filled sufficiently to protect the death spiral of charged and discharged conditions. Put a VSR (Voltage sensing relay) between the house battery he is going to use (abuse) and the backup safety battery. This gives him the ability to use light and hot water to repair the raccoon attack damage. Great battery info on this site http://www.australiandirect.com.au Not a sales pitch as they don't sell out of AusieLand

 

[BigSwede]

.
50% average discharge = 1000 cycles
70% average discharge = 650 cycles
95% average discharge = 400 cycles
.
Now let's say I want a camper that has a useable 255 amp hour battery bank. Traditional logic (and many folks here on expo) would say that I should buy TWO 255ah 12v batteries, or double the capacity I want to use, and never exceed 50% discharge. In this case, total cost for the batteries would be $1300. Total weight would be 312 pounds (each battery weighs 156#). They would last me for 1000 cycles, or 500 cycles PER battery.

Why did you divide the 1000 cycles in half? If each battery is discharging to 50% each time, they should each do 1000 cycles. Which would impact your math significantly.

 

[AndrewP]

Kind of a silly discussion. If you really want to make the math work for the least outlay of cash, you should be using flooded lead acid batteries.

Example: 4 Group 31 Deep Cycle batteries are $119 each, cutting your upfront investment to less than $500. That would give you 420 Ah and thus close to 210 Ah of useful capacity.

OR: 4 Golf cart batteries from Costco at $80 each would cut your upfront costs even more, to $320 and give you roughly 230 Ah usable capacity. You could replace that battery bank 4 times for the cost of 1 set of Northstar batteries.

Is your goal to save weight, save money or what?

And, not every discharge will be to 50 %, many will be less and a few occasionally more. But for the rare times it's more, you'll be happier discharging cheaper batteries. So you have to decide how much capacity you really need. If you're pushing to the edge of your capacity, you are cutting into the reliability of your overall system. At a 95% discharge it is highly unlikely you will get much use out of any battery. Northstar's 400 cycles to 95% is likely best case with immediate lab like recharging, perfect temperature control, and structured discharge rates. In the real world, all of those factors will go out the window.

 

[Martinjmpr]

Well, if you're talking about 255ah battery banks, that's way over my head but as far as the issue of discharging to 50% vs. discharging to 95% I think there are other issues to consider besides the number of discharge cycles the battery can tolerate.
.
Many of the electronic and electric components we use are sensitive to voltage, so a battery may only be at 40% but if that drops voltage down below 11 then, for example, a fridge might shut off, ruining your food, or another electric doohickey might stop working because of the low or fluctuating voltage. That could have an impact beyond any actual shortening of the life of the battery and would have to be factored in to any calculation as well.

 

[jonyjoe101]

my only advice to the topic of discharging a lead acid beyond 50 percent is not to do it.

I had my kinetik khc2000 which is a high end agm (102 ah) for about 3 years, I always had it connected to solar and rarely took it below 50 percent it would usually be at 12.3 volts in the morning.

Recently I started getting a bunch of 18650 lithium batteries (laptop batteries) to test , I had the chargers for these batteries connected to the kinetik and about 4 or 5 times the kinetik went below 12 volts, as low as 11.5 volts. I didn't think much of it. But that killed the kinetik, the battery would show it was fully charge but once it got to 12 volts, the voltage would drop all the way to 10.5 volts.

If you need the amperage get 2 lead acid, and don't take them below 50 percent. Lithium while very tempting is too expensive and I hear all the time the prices are dropping but currently the price on a 12 volt 100 ah lithium is 1299.00.

I did move to lithium, building my own 94 ah lithium battery out of 168 18650 batteries. Its a 3s 11.1 volt battery pack. And this lithium battery I can take it all the way down to 10 volts without damaging it, and I don't have too fully charge it every day, perfect for solar. This homebuilt lithium was just going to backup my agm, because of the lower voltage, but it has become my everyday battery, everything that worked on the agm, work with this battery. But soldering 168 batteries together is not for the faint of heart.

There's a good video on youtube titled "lithium battery test versus gel" where they test a 100ah lithium against a 100 ah lead acid and also 220ah lead acid battery. I was surprise at the results of how much usable power each battery delivered.The test was worst case where you drain the battery quickly and measure how many amps were delivered before the battery quit. Not the more modest 20 hour rate.

100 hour lithium delivered 94 amps
100 hour lead acid delivered 31 amps
220 hour lead acid delivered 73 amps

The video showed one reason why a large lead acid is better than a small one. Especially on a large load, as the battery voltage drops, the load requires more amps to continue to operate, depleting the battery even faster. The large lead acid can maintain the voltage higher longer.

 

[BigSwede]

If you need the amperage get 2 lead acid, and don't take them below 50 percent. Lithium while very tempting is too expensive and I hear all the time the prices are dropping but currently the price on a 12 volt 100 ah lithium is 1299.00.

Because of the 50% usage recommended for lead-acid, I feel a 50 ah lithium should be compared to a 100 ah lead-acid, cost-wise.

What kind of charging management do you use for lithium?

 

[scott7022]

Has anyone looked at the new FireFly batteries. They are getting some pretty interesting reviews on boat sites. My other reason for considering them is a Vancouver Boat Supply is importing and stocking them. They tend not to import stock heavy stuff, duty is paid at rate on the day, so adds a challenge for retail. They must believe in them to do this as it is stupid to stock stuff that doesn't move quickly. Big cycle numbers and loads of headroom for those emergency days.