# Battery Charge Efficiency

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#### ckmcdonald

##### Member
I'm trying to find information about the energy required to charge an AGM battery. In particular, the efficiency of the charging process. For example, if a fully charged 12V AGM battery has 200W of energy used from it, how much energy applied to the posts of the battery does it take to replace the 200W and bring the battery back to full charge.

It seems this type of info would be readily available for any particular battery or type of battery but I'm unable to find any. Maybe because I'm not familiar with the proper terminology.

Can someone give me a little help finding this info?

Thank
Cal

#### jwt873

##### Member
Battery energy is rated by Amp Hours rather than Watts. An Amp hour is how much current a battery can supply over a certain length of time. A one Amp hour battery can deliver one Amp for one hour, or 1/2 of an Amp for two hours, or 1/4 of an Amp for 4 hours or 2 Amps for 1/2 an hour etc..

So if you have a 50 Amp hour battery and you discharge it at 1 amp for 10 hours, you've used 10 Amp hours leaving 40 Amp hours of capacity left.

When charging, you have to go by the battery manufactures recommended charging current. If a 50 Amp Hour AGM battery can safely take a 10 Amp charge, then it will take at least one hour to restore the 10 Amp hours you used.

I'm using round figures... Charging isn't completely linear. In practice it would be a bit more than an hour. Also, some chargers can't deliver a 10 amp charge. If you have a 5 Amp charger in this example, then it will take 2 hours to restore the 10 Amp hours rather than one.

So, to determine how long your battery will take to recharge, you need to know how much current was being drawn and for how long. You also need to know how much current your charger is capable of producing.

#### mmckenna

##### I ♥ Ø
It depends...

Charger efficiency is one factor you need to consider. If you are using an AC powered charger, you'd need to look at the specific unit, but expect something less than 90%, probably a lot less.

Battery charging can also vary. Depends on a number of factors. How quickly it's charged, temperature, etc.

You'd also need to have some specific point as to when the battery is "fully charged". Since battery capacity reduces over time, this will change.

In basic terms, if you tracked how much energy you remove from a battery, you would need to put that much back in to bring the battery back to the same level. You'd need to figure in the charging efficiency, resistance in the wire, connections, etc.

Low end batteries won't have much information out there. If you are looking at larger battery systems, like those used in telecommunication plants, very large UPS systems, etc. you can find some information, but due to the variables, you are not going to find the specific information you seek spelled out. You'll need to sit down with a good meter and do some calculations and then wing the rest.

Doing the following will reduce some of the losses:

1. You need to design your power cabling to reduce voltage drop. There are formulas that will tell you what size conductor to use to achieve a certain level of voltage drop over a specific circuit length. Voltage drop is the resistance in the wire converting the charge current into heat.

2. All your connections need to be properly done. No cheap wire, no cheap crimp on connectors. Crimp connections should be done with the proper tool. Not the \$15 hardware store "smasher" crimpers. You need to have the specific full cycle crimp tools designed for the wire gauge and crimp you are using.
Soldering the connections help. All connections need to be clean and protected. Stainless steel hardware, no-ox type connection grease, etc.

3. Temperature control. Batteries work better when kept at a constant temperature. Manufacturer will usually recommend something in the 70ºF range. Increasing or decreasing this impacts battery performance, sometimes considerably.

4. Since battery performance decreases with time, you'll need to look at the entire expected life span of the system. Figure in some extra capacity to cover yourself down the road.

5. Charging needs to be done at a specific rate, usually battery ampere/hour capacity over X number of hours. Manufacturer info will give you the recommended charge rates. It'll be usually shown as C/20. C = capacity, 20 will be hours. You may find ratings like C/8 C/10, etc. Charging a battery at a faster rate can damage the battery by overheating it. The heat comes from wasted energy. Wasted energy is your charging current just getting converted to heat.

6. Charge voltage needs to be carefully controlled, usually down to the tenths of a volt. Charge voltage needs to be adjusted based on battery temperature. This is usually referred to as "temperature compensation". You'll find this on large telecom grade DC power plants used in central offices or cell sites. Rare to find it on amateur/hobby grade power supplies/chargers.

This is a pretty big subject. Happy to help out as I can, but I'd need a lot of specifics. Anything you can provide will help.

#### RFBOSS

##### Member
In order for your question to be meaningful you must include a unit of time.

In your example did you draw 200 watts for 1 hour. That would be 200 watt/hrs of energy, but if you drew the 200 watts for only half an hour, that would be 100 watt/hrs of energy used.

For AGM batteries it seems the rule of thumb (and this can vary by manufacturer) is it requires about 105 percent of the energy used.

So if you used 200 watt/hrs of energy from your battery you would need about 210 watt/hrs of energy.

However some battery manufacturers claim as high as 99 percent charge/discharge efficiency.

The best thing to do to ensure the longest life and best performance for your battery would be to refer to the manufacturer of the battery.

You can use watt/hrs. or amp/hrs. whatever is convenient for you as long as you understand the difference.

For example, if you have a 12 volt battery and your are drawing 10 amps of current that is 120 watts and if you did that for 1 hour that would be 120 watt hours of energy used.

Or 10 amp hours used. Of course this does not tell the amount of energy used unless you specify the voltage.

In the example above it was 120 watt hours because the voltage was 12 volts, but it it was a 6 volt battery then it would be 60 watt hours of energy used.

Both amp hours and watt hours are used depending on the application.

And as mentioned above, you need to specify if you are talking just about the energy being replaced in the battery (such as computing charge time of the battery) or the overall energy used to charge the battery, taking into account the inefficiencies of the charging system.

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#### mmckenna

##### I ♥ Ø
Battery energy is rated by Amp Hours rather than Watts. An Amp hour is how much current a battery can supply over a certain length of time. A one Amp hour battery can deliver one Amp for one hour, or 1/2 of an Amp for two hours, or 1/4 of an Amp for 4 hours or 2 Amps for 1/2 an hour etc..

Yes, sort of.
Forgive me for this, but I've got to add a disclaimer.

Battery discharge rates are not linear. The ampere hour rating on a battery are strictly for a specific discharge rate. You may find that a 100AH battery will only provide 100 AH if discharge is done over 8 hours, or something close to 12.5 amps per hour over 8 hours. If discharging faster, the available capacity drops off faster. Discharge it longer and at a lower draw, and you'll see more capacity.

#### majoco

##### Stirrer
NiCads are only about 70% efficient, so they need 140% of the nominal A/hour rating to be fully charged from flat. Often the cells start to get warm when fully charged. The large aircraft batteries that I used to work on were charged at the C/2 rate for two hours (they were 40Amp/Hour batteries and C/2 = 20Amps) then trickled at the C/10 rate (4A/H) for 4hours to avoid the overheating effect - these were wet cells. This was from dead flat as every cell had been shorted for 24 hours. NiCads like being treated hard!

#### ckmcdonald

##### Member
Thanks all for the feedback! I even learned a few things I needed but didn't ask about. To clarify on the particular topic I was asking about, the losses suffered when charging an AGM type battery....

The application is solar. I have studied and understand the energy delivery from the PV panel through the charge controller to the battery. I'm planning to use a MPPT controller and understand the charging steps/profile they use and the published efficiency numbers. I haven't purchased a battery yet but am considering one of the VMax-Tanks. In my calculations I'm attempting to account for the charge efficiency of the battery. I heard once that if say 100W is pulled from a battery (for an hour) that it takes more than 100Whrs from energy to put it back. 30% more was one number I've heard. I've spent a bit of time looking for such published numbers and so far have come up empty handed.

So that was my original question. I know there are literally endless details one could attempt to model here, like the profile of a battery as it ages, or, line losses, etc. etc. But what I was looking for is more of a rule of thumb. Something like RFBOSS provided in his post that states AGM batteries are 99% efficient when being charged (which I assume means when being charged properly by a well behaving MPPT controller).

So are AGM batteries really that efficient when being charged, 99%? If so, that's well beyond the accuracy of my general calculations and I could just call that 100% and not account for any loss of energy due to charging.

Thanks again
Cal

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#### RFBOSS

##### Member
Some additional just general information for you.

It is generally accepted that overall charging efficiency can be over 90 percent, but...

When it comes to charging a battery, the charging efficiency will vary pending on the SOC (state of charge) of the battery. For example a battery may have an overall charging efficiency above 90 percent (from fully discharged to 85 percent SOC), but the incremental battery charging efficiency in general numbers from about 80 percent of full charge to 85 percent of full charge is may only be about 55 percent and may continue to decrease as the SOC increases.

These numbers are general numbers and can vary depending on the manufacture. Specific information could be supplied by your battery manufacturer.

This can be significant in PV systems if the designer expects the batteries to normally be operated with an SOC above 80 percent.

#### prcguy

##### Member
I've used a lot of large flooded, gel cell and AGM batteries over the years and have also ruined quite a few expensive ones. I've learned to under rate battery amp hours and expect extended charge times because going by advertised ratings doesn't always work out in real life.

For example, whatever your battery amp hour rating is, cut that in half when actually pulling power from it. For example, if you have a 100AH battery, consider it a 50AH battery when running your equipment unless you carefully measure its voltage under discharge to avoid going below a critical level that will permanently damage the battery. You might be able to pull 100AH from a 100AH battery, but if its left at 10.5V when your done you will be buying a new battery real soon.

Just last week at the Quartzsite ham convention I was talking to some people in the RV industry and they also consider the battery banks in large motor homes at about half their advertised ratings to avoid their customers killing off their batteries prematurely. They add up all their appliances and calculate how long they can run to about half the advertised capacity before looking to recharge. They also carefully monitor the battery voltage and know what the critical low voltage is for their type of battery.

I also count on charging about 150% of what I've used from the battery and its hard to measure exactly what you have put back into a battery without a computerized AH recording device. You can try and estimate like your charger will charge at a 10A rate and a 4hr charge should be 40AH back into the battery. The problem with that is part way through the charge it starts to taper off and its very hard to track the changing charge current over time. A little overestimating on things like this will help keep you out of trouble when advertised battery ratings don't quite add up.
prcguy

#### Rred

##### Member
ck-
An AGM battery could be the small one in your UPS or emergency lighting (still called "gel" although they haven't been for over a decade) or a bigger deep cycle battery. For a loose rule of thumb, for the "car and larger" size deep cycle AGMs, you need to put in about 110% of what you took out. Depending on all the other good stuff, like temperature, state of charge (the last 5% can take forever to put back in), how you are controlling amperage versus voltage (both matter) and also whether you are using pure DC or pulsed DC. (Pulsed DC actually being more efficient, at least in wet lead.)
AGMs are happiest when they are brought back to 100% SOC, not just 95%. Makes a difference in the overall longevity. So, it pays to check.
Depending on the maker, AGM generally is said to charge safely at up to 25% of capacity ("C") versus 20% for wet lead. Having the proper charger settings matters.

#### ckmcdonald

##### Member
This is granted more complicated than I'm trying to make it. From what I gather there's a general charge efficiency for each of the 3 stages of a AGM charge cycles Bulk, Absorption, Float. I understand that Bulk is pretty efficient but Absorption isn't as efficient.

What I'd really like is the average % efficiency from dead (10.5V) until it enters Float. For the VMax batteries I haven't been able to find this data.

Cal

#### RFBOSS

##### Member
So it is correct to say that you will always run them down to 10.5 volts?

It is difficult to give a general number because it will vary with temperature, charge rate (and profile-charger dependent) and the age/number of charge cycles.

Have you contacted VMax? They have a section on their page for technical questions.

Otherwise pick a number, say 95 percent and call it good. This may not result in a completely charged battery at times and it may not provide the best battery life, but you said that you are looking for an average number.

I found this on another battery supplier's web page...

"'Solar' batteries are deep-cycle batteries which have been optimized by the manufacturers to be able to charge with very little current, and thus take maximum advantage of any available energy. Solar batteries also normally have a very high charge & discharge efficiency of around 90 to 95%."

If VMax will not supply you with information you will have to just pick a number or monitor a number of charge discharge cycles and come up with your own. You can use terminal voltage as an approximate indicator of SOC on new batteries after a few cycles.

I found a number of sites that published generic charge profiles for AGM batteries, but I did not bookmark them (of course this does not tell you charge efficiency) but it could give you a place to start.

Generic profiles are a compromise of charge time, final SOC and life cycles, but again it can be a place to start.

Here is another charge profile that indicates a charge efficiency of 85% to 90%. So again, if VMax will not or can not supply the information, you will just have to pick a number or do some research on your batteries in operation and define your own number.

Maybe you could use a battery manufacturer that makes it easier to obtain information.

Let us know how things work out.

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#### Rred

##### Member
FWIW, if you always run batteries (any kind of batteries including any kind of lead acid) down to 10.5 volts, then you might as well buy alkalines and just throw them away after each use.

Lead acid batteries are at an effective zero state of charge at 11.5 volts, not 10.5. 10.5 counts as "deader than a doorpost" and every battery maker will tell you that is severe abuse, which may reduce the number of charge cycles by 90% compared to recharging them at a 70% state of discharge. Which would be around 12.2-12.4 volts, depending on the battery.

At 70% SOC you can expect 2000 cycles. At 10.5 volts...maybe 50. Probably once 5 or 6, for an SLI battery.

#### jonwienke

Battery energy is rated by Amp Hours rather than Watts.

Actually, total battery energy is rated in watt-hours, which is voltage * current * time.

If you run two batteries in series, the amp-hour rating is the same, but the voltage doubles. Rating in watt-hours will capture the fact that total energy has doubled, amp-hours will not.

#### ckmcdonald

##### Member
Vmax publishes these numbers and I believe the percent discharge is taken of the range between 10.5V and Float, 13.6V. So yes, running the thing down to 10.5V isn't a good longevity idea. But, not many radios I've looked at will even run that low. Most are speced at a low of 15% below 13.6V, 11.73V. If my math is right 11.73V is about 60%.

% Discharge --- Cycles
100% --- 300
75% --- 600
50% --- 900
25% --- 1500
10% --- 3500

I'm attempting to design my system such that it rarely needs to pull the battery down below a 25% discharge. But in emergencies I can go on down to around 11.73V. Doing that occasionally won't matter much.

#### ckmcdonald

##### Member
It's the capacity of the battery and the PV I'm trying to determine, I don't own either yet but the money is set aside and waiting. I have an end-of-life Optima battery I got for free and an el-cheapo 20W PV that clearly isn't even cutting it for my 2m rig and is more of a joke (experiment) than useful.

The more power hungry of the two rigs I want to run on the system is about 19A TX at 100W transmit power. RX is 1.2A. I want to be able to run at least 2 days with no sun, 8 hours/day at a duty cycle of 20TX / 80RX. My math is telling me I need a 125 AHr battery. The rig's drop out voltage is 11.73V so that means I only have 60% of the battery capacity available, or 75 AHrs. If my math is right this should run the radio for about 2 days (assuming the battery is new'ish).

I'm pretty confident I have the numbers of this part pretty close. Feel free to check my numbers and let me know if you think otherwise - I'm all ears.

On the charging side I'd really like to size the PV system so at the annual average days of sunlight for my area (4.92) that it will recharge a 60% depleted battery back to Float in 1 day. Using 92% efficiency for an MPPT controller and a 10% overhead for charging inefficiency (what started this thread) I get a PV of about 200W to charge in one day.

Am I on the right track, or all washed up on my calculations?

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