Depth of Discharge

[tvtech]

Hi Guys

Many terms are bandied about with rechargeable batteries. Deep Discharge to 100% DOD is a term used for deep cycle batteries as used in Golf carts and stuff...

I understand a 12V battery going down to 9.5V probably in Deep Discharge under load.

Problem is how does one work out say 50% DOD with a known battery AH capacity and known load??
I ask this because the Internet wont tell me. I am not acting dumb. I cannot find answers.

Here it is:

1. 65Ah Sealed Lead Acid battery.
2. Constant load say 10 Amps from the battery
3. With the constant and consistent load...at what point should the Battery be shut down because it has reached 50% DOD?
Probably a huge question...I need to know the Voltage to shut off. Could be 12V or maybe 11.5V or maybe 11V.

No games here. A serious question.

Regards,
tv

 

[audioguru]

I have never used a lead-acid battery. My car has one.
Whenever a car battery has been allowed to completely discharge or partially discharge then it is doomed and will completely fail soon after. A Deep Discharge battery is different.
www.batteryuniversity.com tells you all about them.

 

[MikeMl]

Here is a wealth of info. **broken link removed**.

See if you can get a maker's data sheet fro your specific battery.

OTTOMH, 11V is too low for even 100% DOD.

 

[tvtech]

Hi AG

Not a car. Seriously a question. The Internet talks about "depth of discharge".....but nobody really understands it.

Regards,
tv

 

[tvtech]

And you have Forums out there that teach all necessary about Batteries and how to care for them..

Like big heavy expensive setups...advise you against going below 50% DOD...but do not tell you how to Monitor stuff...so you don't go below 50%..

Here is one: https://www.otherpower.com/off-grid-battery-guide

So, I am stuck.

Regards,
tv

 

[JimB]

There seems to be a lot of contradictory information about battery voltage vs state of charge for lead acid type batteries.
As has already been suggested, if you can get the data for your specific battery from the manufacturer, that would be the best way to go.

Other than that, just use the info in your "otherpower" link and call it done.
Otherwise you will be dithering around deciding what to do and getting nowhere.

JimB

 

[barshad]

Hello

 

[tvtech]

Hello

Hello

Are you well??

 

[tvtech]

There seems to be a lot of contradictory information about battery voltage vs state of charge for lead acid type batteries.
As has already been suggested, if you can get the data for your specific battery from the manufacturer, that would be the best way to go.

Other than that, just use the info in your "otherpower" link and call it done.
Otherwise you will be dithering around deciding what to do and getting nowhere.

JimB

Hi Jim

The problem is what is 50% DOD on a specific battery...
Nobody knows. Like a pie in the sky...

100% discharge is easy to work out...normally down to 9.5V.

Outta here,
tv

 

[ronv]

Here is one.
Edit:
Did I tell you nice avatar??

 

[Cicero]

Hello

Hello

Are you well??

Hahahaha

Tvtech, are you gearing up for load shedding or something?

 

[tvtech]

Here is one.

Thanks Ron

Exactly what I was looking for.

Regards,
tv

 

[tvtech]

Hahahaha

Tvtech, are you gearing up for load shedding or something?

Yip Cicero. Eskom is so up and down at the moment...you never know what might happen next

Stay well fellow countryman.

All the best,
tv

 

[MrAl]

Hi,

Ampere hour capacity varies from battery to battery, even of the same make and model. When new it should deliver the stated capacity and the test loads will show up on the data sheet for that battery. For a given load, you should be able to find out how long you can run that load to get down to 50 percent capacity. The time will vary with load and it is not linear at all but follows an exponential law, but looking at the data sheet they should show different capacities at different load currents.

As the battery ages however, the capacity does down. That means if when new you could load it for 32 hours with 1 amp to get to 50 percent, you may have to cut that back to 25 hours for example when it is a couple years old. You may be able to get some idea what to expect from the data sheet by looking at the capacity with aging graph if they provide one. If not, you may have to just wing it.

While the battery is powering something you can integrate the current over time to see how much capacity has been used so far. For a constant current this is easy. Once you establish an operating set, you could probably go by that for the future use of the battery. In other words, get the 50 percent capacity voltage from the data sheet and time how long it takes to get to that voltage with your given load. Each time you recharge, start the timing procedure over again, so if it took say 20 hours to reach 12.2v when new and that was 50 percent capacity, then note how long it takes to get to that same voltage each time the battery is used. If it gets down to 12.2v in only 10 hours after several uses, you know the battery is getting old.
The key here though is the data sheet, for the first measurements. After that it's just a matter of timing the discharge to the same point each time it is used.

I think the only truly reliable method for estimating charge without lots of battery data is to measure the specific gravity of the electrolyte. That means you need an open cell top battery. The relationship between specific gravity and state of charge is very accurate.

 

[tvtech]

Hi MrAl

ronv gave me the info I was looking for. Could not find the graph on the Net. Thanked Ron and gave him a "like".

I am sure you remember me and my PP3's....long ago. Never lost one yet. Fragile things that are all still alive.....two worn out but OK...changed them anyway. They went through hell with testing. Not one failure in what five or six Years now...

I understand a PP3. Got it right first time round.

SLA and all and Inverters and charging is another story. Big money you are playing with. And lots of energy.
My second time around doing this....

Cyclons back in 2005 here. When they are flat or have worked or need amps to charge...you can literally throw the kitchen sink at them as far as Current goes.....they need no Current limiting. You have 100A at your disposal. If hungry they will eat it no sweat. Just keep the Terminal Voltage stable.

Amazing batteries. Charge very quickly and are deep discharge too. And expensive. And a Battery Charger that can do what they can consume is hectic money....and I am talking about an 8aH battery. Strictly Military or Medical type batteries....good learning curve for me though.

So, learning from my previous mistakes and all, I am going another round.

Looking for storage capacity this time and how to treat SLA well....get the most from a battery without going down to 100% DOD.

Fundi's who know stuff say below 50% DOD is where you should never go. No matter Deep Discharge or whatever.

They also talk about Batteries suffering more from undercharging problems that kills them over time than being a little overcharged. So much to learn still.

I think you have to understand what you are are dealing with.

Little PP3's were easy. This is another animal.

Regards,
tv

 

[MikeMl]

The most accurate method of determining SOC for LA batteries is Specific Gravity, which you cant measure in a Sealed Battery.

Second most accurate is to measure the battery's open-circuit terminal voltage 24 hours after the battery is isolated from a load. Hard to do if the battery is being discharged...

The voltage as a function of load current and time is the next best thing, like in the link I posted...

However, Ron's and my plots are examples only; you need to get one for the specific make/model of the battery you are using...