Another view of the 50% AGM Discharge Limit

[booster]

There have been a couple of discussions that had some Smartgauge questions and information in them, and Smartgauge addresses the 50% rule as follows:

2. Assume the amp hours counter is set up for a 400 amp hour battery bank. The 50% rule tells us we should be discharging to -200Ahrs on average, then recharging. (see the 50% rule). Regularly discharging below this level unacceptably shortens the life of the batteries and reduces their capacity. Just one or two discharge and recharge cycles below this level can reduce the capacity of a battery bank by 1%. So after 5 discharge and recharge cycles, this 400Ahrs capacity battery bank could actually be a 395Ahr battery bank. Discharging to -200Ahrs is now below a 50% discharge. So this has more and more effect on reducing the capacity of the battery bank each time it is cycled.

If you compare what they say to what the charts are showing, it is hard to see where they could get the statement amount loosing 1% of bank capacity by going under 50% for one or two cycles. That would mean if you had between 20 and 40 cycles under the 50% SOC point, you would be at the 80% of starting capacity that indicates a worn out battery by the manufacturers specs. That is just plain silly, IMO, and would probably not even happen if you took them dead 20 times, as long as you got a timely, complete, recharge.

In other places in the tech literature, Smartgauge lists cycles and recharges, but in most cases where the actually say anything about it, they are recharging to 4% of capacity for float transition amps, which is way high, and would explain why they say they never get back to 100% after any cycle. That will cause more capacity loss than discharging below 50% IMO.

The deeper I read into the Smartgauge tech pages, the less impressed I have gotten. They roll out how many millions of calculations they do, and spend pages showing equations for Peukert and other things, but then totally miss how Peukert really works and don't say anything (at least that I saw) about charging batteries to an amp level, which is what the manufacturers recommend. This is probably because their algorithm does not match amp triggered full battery criteria, and they don't measure amps anyway.

[booster]

There were also some questions about how Peukert figured into the 50% rule in this thread. Based on the discussion we had in this thread:

http://www.classbforum.com/forums/f5...tood-3770.html

It would appear the Peukert shouldn't really have any effect on the actual amount of energy left in the battery at various discharge rates. The only time the higher discharge rate comes into play is right at the end of a deep discharge cycle, and are going to voltage reading, not and amp hour reading like a battery monitor will give you.

[markopolo]

Some of the info on the Smartgauge site does go against what we've learned.

[WJones]

On the Smartgauge site, I found their explanation of the "50% rule" confusing to say the least. At one point, I wasn't sure whether they were explaining the "50% rule" or Peukert's law. It seems to me explaining it could have been a lot simpler.
Now, with all these sites pushing the same story, demonstrating the fact that this "rule" is in fact a "myth" is an uphill battle.
If I met a neophyte who was getting all wound up hearing that he's killing his batteries by discharging them 51%, I would tell him that what follows is the rule he should follow:
"Charge your batteries properly and do it as quickly as possible after discharging them. Forget the rest and enjoy your camping trip."

[booster]

Originally Posted by WJones

On the Smartgauge site, I found their explanation of the "50% rule" confusing to say the least. At one point, I wasn't sure whether they were explaining the "50% rule" or Peukert's law. It seems to me explaining it could have been a lot simpler.
Now, with all these sites pushing the same story, demonstrating the fact that this "rule" is in fact a "myth" is an uphill battle.
If I met a neophyte who was getting all wound up hearing that he's killing his batteries by discharging them 51%, I would tell him that what follows is the rule he should follow:
"Charge your batteries properly and do it as quickly as possible after discharging them. Forget the rest and enjoy your camping trip."

Very will stated! Timely and accurate charging, IMO, is much more important than not going under 50%. Unfortunately, most of the chargers don't do a very good job of getting totally full, but I hope that will change as more people get aware of the issue.

Maybe it is time to start carrying a copy of the "lifetime battery amp hours" graph to show the skeptics!

[Crank]

Originally Posted by WJones

Totally agree with your comments. Your comment about the limited space in a class B hits home in my case. I have a 105 Ah battery and if I want to use 80 Ah before recharging, and I follow the 50% "rule", then I absolutely need a second battery. In my Roadtrek, this would be a major undertaking to do it right. 2000C200P

Two batteries in a Roadtrek, a circuit and a system that FULLY charges them.

Observation:

How long a battery lasts (for 1 charge, and for its lifetime) depends in part on how fully it is routinely charged: often 100% full is not possible. This fact bears on the efficiency discussion.

Adding batteries to a small Class B:

I have just installed TWO 220 Ah 6v Lifeline GLP-4CT AGMs in one of the smallest, fully self-contained vans out there at under 20', my 1995 Dodge Roadtrek 190 Popular, during a complete 12v power system upgrade, WITHOUT losing any storage. Differing floor plans and vans may demand more radical solutions than my simple one.

Immediately in front of the Onan in the left rear corner under the bed /couch is the power section. The obsolete, battery-eating Magnetek charger in it was replaced by a 3 bank 40A BlueSea (BS) P90 (hung on a closet wall to free compartment space). A Trimetric 2030-RV monitors the batteries, wired as a single bank) and % full, independently of the BS.

One battery went in the original battery box adjacent to the Onan. After removing the obsolete Magnetek charging section (the 3 only wire cuts made), slightly relocating other AC gear & its snakes-nest of cables (without actually disconnecting anything), the 2nd battery fit next to the 1st, between the wheel well and the Magnetek distribution panel (moved 3" into the aisle, no problem). 2 more batteries could have gone in a closet next to the wheel well.

Attaining a Full Charge:

It is well known that a temperature-compensating charger programmed to the battery maker's specifications is essential (but not sufficient) to maximize the charge and limit battery damage. The BS charger meets these criteria. However, a concurrent load interferes with the charger's sequence and denies a full charge. This is of special concern in a small battery system.

Booster recently introduced an ingenious, but inexpensive, easily made circuit to enable the BS to ensure a FULL charge in spite of a concurrent load. (I incorporated his circuit in the lower part of the distribution panel case.) More on the Booster circuit
http://www.classbforum.com/forums/f8/temp-compensating-charger-dp-paramode-4100.html . (Big thanks to Booster for giving me the circuit, and more importantly, walking me through my installation, without which I could not have offered this contribution.)

During charging, the Booster circuit allows the primary BS charging circuit to continue charging the single battery bank until they reach 100% charge (as accurately displayed on the Trimetric), but switches all active loads (e.g. refrigerator, monitors) to the second BS charging circuit which is unconnected to batteries and feeds concurrent loads directly. One reason this system works well is that the BS defines a battery as fully charged by amps which is more accurate than using voltage.

The Trimetric is essential, especially with the Booster Circuit, because it operates independently of the BS, accurately measuring volts & amps flowing to the batteries and most importantly, displaying their state of charge. It flashes at 100%Full, warning overcharging is imminent if charging power from external 12v sources (i.e. an alternator/separator system or generator) is not interrupted. (Chargers cannot control 12v input because it does not flow through them). The Trimetric also stores data on power use and charge cycles until it can be logged for efficiency and cost analysis.

This system with the Boooster circuit is a great upgrade for smaller RVs that need the highest efficiency and capacity, especially if they have poorly-matched chargers. Its limiting factor is its 40 amp power supply: the more batteries, the longer the BS will take to charge them. I imagine 480-600Ah would be about the practical limit. depending on how deeply they are routinely drawn down and how quickly they must be recharged. (Weight and space also become important considerations relative to RV size.) Few if any other chargers provide the flexibility of the Blue Sea at a moderate price, taking into account ruined batteries, relative to the cheap, poorly controlled chargers used by much of the public (including me before I got educated)
Crank