Battery Depth of Discharge question

[pellicle]

Hi

pardon if this is the wrong place, but I wanted to ask about measurements of 12V batteries for DoD. I understand (and have read a few tables) that there are standard voltages for measuring DoD based on the 12v cell at "no load" state (I assume an amount of time to settle such as 30min).

I've read here (http://www.solar-electric.com/deep-c...ttery-faq.html at state of charge) that something like 12.06V is about the 50% , however I'm wondering if there is a way to measure this when the battery is under a load (something light like 250mA and the battery is 8AH) .

Of course I can do charts by running to a voltage measuring that, unloading and waiting 30 min and measuring again. I would wonder if that's reliable.

Any inputs would be welcomed

Thanks

[inetdog]

Hi

pardon if this is the wrong place, but I wanted to ask about measurements of 12V batteries for DoD. I understand (and have read a few tables) that there are standard voltages for measuring DoD based on the 12v cell at "no load" state (I assume an amount of time to settle such as 30min).

I've read here (http://www.solar-electric.com/deep-c...ttery-faq.html at state of charge) that something like 12.06V is about the 50% , however I'm wondering if there is a way to measure this when the battery is under a load (something light like 250mA and the battery is 8AH) .

Of course I can do charts by running to a voltage measuring that, unloading and waiting 30 min and measuring again. I would wonder if that's reliable.

Any inputs would be welcomed

Thanks

First of all pellicle, welcome to Solar Panel Talk!

If you have flooded lead acid (FLA) batteries then the only reliable way to estimate SOC is from a temperature corrected hydrometer reading. If you have sealed (not just maintenance free) batteries, then you are out of luck on that.

You are on the right track with need for taking the resting voltage, but the time is more like three or four hours than just 30 minutes.

You can also find some very general curves for voltage under load or charging versus SOC, with different curves for different current as a fraction of C₂₀, the amp-hour capacity at the 20 hour rate. But if you want to actually use those curves you need to calibrate them for your own battery, by starting with a known SOC and measuring the voltage as a function of charge or discharge current.

For a generic FLA battery 12.06 volts is closer to 25% SOC than 50% SOC. But that depends strongly on the chemical composition of the plate material too.
(Pure lead, lead-calcium, lead-antimony, etc.)
Check out http://batteryuniversity.com/learn/a...tate_of_charge for an overview.
The articles at batteryuniversity.com are considered pretty authoritative.

[pellicle]

Hi

First of all pellicle, welcome to Solar Panel Talk!

Thanks

If you have flooded lead acid (FLA) batteries then the only reliable way to estimate SOC is from a temperature corrected hydrometer reading. If you have sealed (not just maintenance free) batteries, then you are out of luck on that.

Its a AGM, so I fall into the SOL category there ...

but the time is more like three or four hours than just 30 minutes.

OK ... good to know. I guess battery physical volume makes an impact there.

Check out http://batteryuniversity.com/learn/a...tate_of_charge for an overview.
The articles at batteryuniversity.com are considered pretty authoritative.

Thanks, have used them a bit, but in a patchy way.

Thanks again for the inputs. I'm planning a solar system to pump water through a solar collector and under my floor to heat the home.

[Sunking]

The issue with voltage SOC is a batteries internal resistance. As current increases through a given resistance, the voltage drop increases Voltage = Current x Resistance. So for example take say a FLA 12 volt 100 AH battery. On average the internal resistance of said battery is on the order of .02 Ohms. Sound low right? Wrong!

So assume this battery is fully charged and shows a Open Circuit Voltage of 12.6 volts or 100% SOC. Now put a light load of say 12.5 amps (C/8) onto the battery. What happens to the voltage?

It drops from 12.6 volts down to 12.6 - [12.5 amps x .02 Ohms]= 12.35 volts. Even though the battery is in reality 100% SOC the voltage under load indicates 70% SOC or a 30% error. Increase the load current to 100 amps or a 1C discharge rate and the voltage drops to 10.5 volts and your inverter drops off line. Even though the battery is 100% SOC, the voltage indicates it is a boat anchor and 100% error.

[mapmaker]

For a generic FLA battery 12.06 volts is closer to 25% SOC than 50% SOC. But that depends strongly on the chemical composition of the plate material too.

Here's a chart from a Trojan Battery user manual:
trojanSOC.jpg

Note that 12.06 volts is very close to 50% SOC.

--mapmaker

[inetdog]

Note that 12.06 volts is very close to 50% SOC.

--mapmaker

And at the link i gave from battery university, 12.06 is shown as 25% SOC for a Lead-Antimony formulation (maybe more common for starting than for RE use?)

[PNjunction]

For more accurate results when using AGM, the following may be useful:

http://www.bdbatteries.com/mcharging_procedures.php

Note that these are specific to Concorde / Lifeline agm's, but are pretty close to most other agm manufacturers. It can be a subtle distinction between flooded, but you'll notice about a 0.2v difference, which is what it should be since agm's have an overall higher voltage due to lower internal resistance.

If you really want to get close, the manufacturer may have even more precise charts.

Unfortunately, being sealed with no access to the individual cells, there is no way to determine if those cells are well balanced, further masked by the fact that we don't take batteries below a 50% DOD anyway.

[pellicle]

Thank you all for the good responces, it has been a good learning exersize.

This reply was also useful and leads me to a question

For more accurate results when using AGM, the following may be useful:

http://www.bdbatteries.com/mcharging_procedures.php

Looking at those numbers it would seem that my battery will never be treated properly as its in my motorbike, there the regulator seems to always give it 14v

My guess is that this means that the battery will suffer on longer trips (many hours).

Do you have any views on that? I guess its just a matter of chalk it up to "wear and tear"

Also, when charging it recently I was monitoring it every hour and found that on my last inspection it had reached 60°C (measured with a IR type thermometer). Does this mean that I've likely cooked it?

PS: one of the other batteries (same model battery) is being used to power the water pumps on my solar floor heater. Its basically a black poly pipe collector which then runs into a collector box and then under the floors. Queensland doesn't get real cold but it makes a nice improvement to the house being comfortable.

I'm currently trying to work out if its worth powering the pumps by solar PV and the battery or just going over to mains current for the pumps, which are after all only a few hundred mA each (although there are currently 6 pumps).

Thanks

[PNjunction]

Looking at those numbers it would seem that my battery will never be treated properly as its in my motorbike, there the regulator seems to always give it 14v

That might be solely at idle, you need to check at a more normal running rpm. And do that at the battery terminals, not relying solely on any external gauge. It could also be that your regulator is fried, and while putting out 14v at idle, runs up to 15v or more during running rpm's.

My guess is that this means that the battery will suffer on longer trips (many hours).

Depends. That might be the reason for the low 14v. With a small 8ah agm, then what would appear to be an "extended float", at that voltage won't kill the agm immediately. Or, your bike may have originally been intended for gel, and not agm.

Also, when charging it recently I was monitoring it every hour and found that on my last inspection it had reached 60°C (measured with a IR type thermometer). Does this mean that I've likely cooked it?

Yes, assuming your IR thermometer can be trusted. Most agm's especially small ones can't take anything much over 130-150C. Is 8ah the capacity originally spec'ed for your bike? Or it could be just a case of a poor agm brand, coupled with bad/failing charging techniques, high resistance wiring or a bad parasitic load like alarms and whatnot, has taken it's toll.

My suggestion would be to start over with a fresh agm, of high quality like a PowerSonic, Yuasa, or Enersys genesis (conventional, not the tppl type yet). Check your voltages under different rpms.

I'd also highly recommend something you should probably be able to find in Queensland, like a CTEK MXS 3.8 charger. This would be great for the bike and possibly those pool pump batteries, IF they haven't already been hurt too badly. Conveniently has a low current setting "motorcycle" I believe and does a few other tricky automated things. Has specific settings for agm or flooded. Safe to leave on the bike.

[pellicle]

Hi, and thanks for your reply

That might be solely at idle, you need to check at a more normal running rpm.

understood ... its 13.99 at idle and moves quickly up to 14.25 as the RPM goes up (digital MM, Fluke)

With a small 8ah agm, then what would appear to be an "extended float", at that voltage won't kill the agm immediately

nice to know.

Or, your bike may have originally been intended for gel, and not agm.

well as it happens I am almost certain that the charging system dates from back in the lead acid days. The regulator is the same part number as that in other earlier bikes.

Yes, assuming your IR thermometer can be trusted.

my experience is that its quite close, certainly within 5C

Most agm's especially small ones can't take anything much over 130-150C.

ok, so that's well over the point I went to (being 60C) ... so probably its not cooked

Is 8ah the capacity originally spec'ed for your bike?

it is ...

Or it could be just a case of a poor agm brand,

that's possible, which is why I was testing it and asked the initial questions.

coupled with bad/failing charging techniques, high resistance wiring or a bad parasitic load like alarms and whatnot, has taken it's toll.

as it happens there is a parasitic load which occurs from engine shutdown to the time when the coolant reaches 60C ... this is caused by a funky design where the auto chokes (solenoids) are powered till it cools down (don't ask...)

I've measured it and found that the load varies from 400mA at key off - to about 900mA by the time the thermoswitch cuts out. Duration is about 45 minutes. Given a couple of 15minute rides during the day I believe that the charging circuit can not balance that out.

I don't know how many amps go into the battery (and for how long) under ride conditions, but I'd think its no more than 2 or 3 ... so at (splitting that) 5AH (charging during ride) I don't know that it'll balance the nearly 1AH discharage.

I'd also highly recommend something you should probably be able to find in Queensland, like a CTEK MXS 3.8 charger. This would be great for the bike and possibly those pool pump batteries, IF they haven't already been hurt too badly.

thanks, I'll look that one up My current battery charger is an oldie but seems to be constant V and therefore varied amps

I'm just getting my head around the notes I made from other posts here about calculating battery internal resistance. Its a topic I've never put much effort into nor been shown how to calculate it.

Thanks again

[pellicle]

charging data question

Hi

The battery has been resting for about 4 hours now and I put it into the charger.

It was 12.45v at rest
With my DMM in line with the -ve I see 1.6A going into the battery. Placing the charger back on and measuring the voltage across the terminals (with the charger on) its 14.47v

Am I calculating right that the internal resistance is 1.26 ohms? Battery is rated at 8AH

If so I expect that this is too high and indicates the battery is cactus