quick question regarding aH ratings of batteries. 20aH rating / 100aH rating??

[urbang33k]

With the help of WWW in a previous thread of mine it was suggested that for my 260watts of panels I should have about 260 aHs of batteries. great!

So i set about shopping away for batteries but I'm finding that the aH rating are based on 20aH or 100aH with sometimes very different values for each.

I feel compelled to ask before I make a bad setup worse. Do I buy 260 aH's of batteries based on their 20aH rating or 100 aH rating? I'm going to guess it's probably the 100aH rating but would like confirmation on that.

Thanks guys!

eg: http://www.heshomeenergy.com/dekabatteries.html

[Naptown]

With the help of WWW in a previous thread of mine it was suggested that for my 260watts of panels I should have about 260 aHs of batteries. great!

So i set about shopping away for batteries but I'm finding that the aH rating are based on 20aH or 100aH with sometimes very different values for each.

I feel compelled to ask before I make a bad setup worse. Do I buy 260 aH's of batteries based on their 20aH rating or 100 aH rating? I'm going to guess it's probably the 100aH rating but would like confirmation on that.

Thanks guys!

eg: http://www.heshomeenergy.com/dekabatteries.html

Most batteries are sized on the 20 hour rate meaning the load would take 20 hours to discharge the battery.
When you apply a large amp draw on a battery the capacity is diminished Which is why a battery has a capacity o say 100AH at the 20 hour rate and maybe 25 at the 5 hour rate. The reason is Peukerts law.
So to accuretly determine the amp hour capacity of a battery bank you need to take the current draw at maximum draw and compensate if necessary.
So if you have a 100AH battery at the 20 hour rate and 50AH at the 10 hour rate the battery would have half the AH capacity when drawing at the higher load.

Don't consider the Gel batteries in that link they tend to be more of a starting battery and not well suited to solar.

[inetdog]

With the help of WWW in a previous thread of mine it was suggested that for my 260watts of panels I should have about 260 aHs of batteries. great!

So i set about shopping away for batteries but I'm finding that the aH rating are based on 20aH or 100aH with sometimes very different values for each.

I feel compelled to ask before I make a bad setup worse. Do I buy 260 aH's of batteries based on their 20aH rating or 100 aH rating? I'm going to guess it's probably the 100aH rating but would like confirmation on that.

Thanks guys!

eg: http://www.heshomeenergy.com/dekabatteries.html

Hi Urban,

Little details matter in this business. 260aH means 260 Amp Hours, which is the product of the amps pulled from the battery by the time that the current is being pulled.

Batteries will convert chemical energy to electrical energy more efficiently at low amperage.
So the 20 hour (not amp-hour) measurement tells you (indirectly) what current the battery can deliver constantly for 20 hours and be totally drained at the end. You multiply that rate by 20 to get the 20 hour rating. So when you see the 20 Hour Amp-Hour rating, you can divide it by 20 to see what current the battery can deliver for 20 hours, ending with complete discharge.

The 100 hour measurement pulls a smaller current which will drain the battery in 100 hours. Then that current times 100 hours is the amp-hour rating.

For most battery PF installations, you will be drawing power from the batteries at about the 20 hour rate, which when you stop after only drawing one-fifth of the battery capacity means you will have that current for 4 hours. Or a total of 45 hours of intermittent use.

If you are actually going to drain the battery at the smaller 100 hour rate, (giving you 20 hours of total use while being kind to the battery), then the 100 hour capacity is the more important one.

For a given type of battery (chemistry, quality, and mechanical construction) the two figures will be in about the same proportion for all of them.

[Naptown]

Hi Urban,

Little details matter in this business. 260aH means 260 Amp Hours, which is the product of the amps pulled from the battery by the time that the current is being pulled.

Batteries will convert chemical energy to electrical energy more efficiently at low amperage.
So the 20 hour (not amp-hour) measurement tells you what current the battery can deliver constantly for 20 hours and be totally drained at the end. Then multiply that rate by 20 to get the 20 hour rating.
The 100 hour measurement pulls a smaller current which will drain the battery in 100 hours. Then that current times 100 hours is the amp-hour rating.

For most battery PF installations, you will be drawing power from the batteries at about the 20 hour rate, which when you stop after only drawing one-fifth of the battery capacity means you will have that current for 4 hours. Or a total of 45 hours of intermittent use.

If you are actually going to drain the battery at the smaller 100 hour rate, (giving you 20 hours of total use while being kind to the battery), then the 100 hour capacity is the more important one.

For a given type of battery (chemistry, quality, and mechanical construction) the two figures will be in about the same proportion for all of them.

No the 20 hour rate is the amp hour capacity of the battery when the load is drawn and battery is depleted after 20 hours
Divide the AH capacity by 20 to arrive at the amp draw to allow for that capacity. Which means that on a 12V 100AH battery at the 20 hour rate = a load of 60Watts (100/5)*12=60 or 5 amps.
Same battery at the 100 hour rate 100/100*12= 12W or 1 amp.
Now lets imagine you are using a 100 A battery @ 12V you have an inverter hooked up to them drawing 300 Watts @ 120V
Without taking inverter losses into account or power factors that 300 Watts @120V = 300Watts at 12V
So 300/12=25A That 100 AH battery now has a capacity of 62 AH and will only last for a little over 2 hours till dead and damaged.
I you want to stay in the dod range of 20% you could only run that load for 1/2 hour
Same 300W load at the 20 hour rate would require a battery with an AH rating of 500 AH
You can back into this by multiplying the draw in amps *20 In this case 25A*20=500AH

So load is equally as important as total watt hours.

[Sunking]

OK let's see if we can clear some things up. What is throwing everyone which has not been Mentioned yet is Peukert Law. This is why there are several ratings for the same battery.

Peukert Law simply states: The rate at which a battery is discharged. As the rate increases, the battery's available capacity decreases.

So what you will see in the better quality batteries they will specify the AH Capacity and various discharge rates. 20 Hour discharged rate is the one we specify and use for RE calculation. I will show you an excellent example of a very high quality battery and Peuket Law in effect. Click here for the Spec Sheet of a Rolls S-460 which is called a 6 volt 460 AH battery. The 460 AH rating is at the 100 Hour discharge rate. So that means you can draw 4.6 amps for 100 hours before it is completely discharged. No scroll down to the 20 Hour discharge rate and note it is now a 350 AH battery. So at 350 AH the metric changes to drawing 17.5 amps for 20 hours. Now go all the way to the 1 hour discharge rate and note it is now a 126 AH battery. So for the same battery the AH capacity is 126 AH to 460 AH.

That my friends is Peuket Law 101. Since it is a Law of Physics, you cannot break the LAW, it will break you if ignored.

[urbang33k]

20 Hour discharged rate is the one we specify and use for RE calculation.

First thank you guys for the very detailed explanations. I'm learning a bit here.

So given that the general rule of thumb is = 1watt of panel to 1 aH of battery when sizing a battery bank and given that I already have my 260 watts of 12V panels making approx 15 amps of charge........

My goal is to select batteries now to replace my 12V marine batteries I bought. (see my other thread)

I'm looking for 260 aH worth of batteries (as per the general rule of thumb mentioned earlier in my other thread) and I'm selecting batteries based on their 20 aH rating?

so purely as an example from the link in the first post..... see the table that claims the flooded AP-27 battery with 90 Ah @ 20 Ah rating , I should be able to paralelle 3 of those guys together for 270Ah total?


Why do the gel batteries listed in that link have such poor 20 Ah rating as compared to their 100 Ah ratings?

[inetdog]

First thank you guys for the very detailed explanations. I'm learning a bit here.

So given that the general rule of thumb is = 1watt of panel to 1 aH of battery when sizing a battery bank and given that I already have my 260 watts of 12V panels making approx 15 amps of charge........

My goal is to select batteries now to replace my 12V marine batteries I bought. (see my other thread)

I'm looking for 260 aH worth of batteries (as per the general rule of thumb mentioned earlier in my other thread) and I'm selecting batteries based on their 20 aH rating?

so purely as an example from the link in the first post..... see the table that claims the flooded AP-27 battery with 90 Ah @ 20 Ah rating , I should be able to paralelle 3 of those guys together for 270Ah total?


Why do the gel batteries listed in that link have such poor 20 Ah rating as compared to their 100 Ah ratings?

You are welcome, that is why this forum is here.

The rule of thumb of 1 AH of battery to 1 watt of panel applies strictly only when the battery voltage is ~12 volts. There are two reasons for it:

1. The panels have to have the ability to deliver a large enough current to keep the battery happy. If you have FLA, that amounts to roughly the AH capacity of the battery divided by 12, regardless of how many sun hours you have and how heavily you load the batteries. This gives a MINIMUM size of panel to keep the battery healthy. Buying more battery than that (at least using FLA batteries) will shorten the battery life. Note a current of Battery AH/12 gives the same numbers as just setting the panel watts equal to the battery AH if the battery is 12 volts.
2. The other consideration is what load you need to power. This tells you directly how large a battery pack you need, independent of the size of your panels.
You calculate how large a battery you need to produce the power you need, taking into account sunless days.
So really, the sizing of your system should start with the load. That tells you how much battery storage you need.
Then, knowing that, you can calculate how much panel you need to recharge the battery fully before the next period of use. But even if you use the power for only one day and then give it six days to recharge, the first condition puts a minimum on the amount of panel you need.
For a "typical" 12 volt installation, which never matches anyone's real use case, the load calculation often happens to come out with a number close to the one from the watt to AH "rule". But that is just coincidence. You need to do the real math for your planned use instead.

Regarding the difference between 20 and 100 hour capacity: A battery with high internal resistance will be less able to deliver its full stored energy at high currents, so the difference will be greater. Things like the geometry of the plates, the relative amount of electrolyte and the freedom of the electrolyte to move all affect this. The numbers in Peukert's law for a battery are an experimentally derived figure that describes the actual results based on all of these factors and more.

[Naptown]

OK let's see if we can clear some things up. What is throwing everyone which has not been Mentioned yet is Peukert Law. This is why there are several ratings for the same battery.

Peukert Law simply states: The rate at which a battery is discharged. As the rate increases, the battery's available capacity decreases.

So what you will see in the better quality batteries they will specify the AH Capacity and various discharge rates. 20 Hour discharged rate is the one we specify and use for RE calculation. I will show you an excellent example of a very high quality battery and Peuket Law in effect. Click here for the Spec Sheet of a Rolls S-460 which is called a 6 volt 460 AH battery. The 460 AH rating is at the 100 Hour discharge rate. So that means you can draw 4.6 amps for 100 hours before it is completely discharged. No scroll down to the 20 Hour discharge rate and note it is now a 350 AH battery. So at 350 AH the metric changes to drawing 17.5 amps for 20 hours. Now go all the way to the 1 hour discharge rate and note it is now a 126 AH battery. So for the same battery the AH capacity is 126 AH to 460 AH.

That my friends is Peuket Law 101. Since it is a Law of Physics, you cannot break the LAW, it will break you if ignored.

See Post #2&4

I have learned a bit from you
Tried to post a Peukerts law calculator but I couldn't attach

[inetdog]

So purely as an example from the link in the first post..... see the table that claims the flooded AP-27 battery with 90 Ah @ 20 Ah rating , I should be able to paralelle 3 of those guys together for 270Ah total?

Whenever possible plan to put your batteries in series. They will last longer and give better service.
Do this even if you have to get 2 volt, 4 volt or 6 volt batteries of sufficient AmpHour capacity so that you can put them in series instead of putting 12 volt batteries in parallel.

[urbang33k]

You are welcome, that is why this forum is here.

The rule of thumb of 1 AH of battery to 1 watt of panel applies strictly only when the battery voltage is ~12 volts. There are two reasons for it:

1. The panels have to have the ability to deliver a large enough current to keep the battery happy. If you have FLA, that amounts to roughly the AH capacity of the battery divided by 12, regardless of how many sun hours you have and how heavily you load the batteries. This gives a MINIMUM size of panel to keep the battery healthy. Buying more battery than that (at least using FLA batteries) will shorten the battery life. Note a current of Battery AH/12 gives the same numbers as just setting the panel watts equal to the battery AH if the battery is 12 volts.
2. The other consideration is what load you need to power. This tells you directly how large a battery pack you need, independent of the size of your panels.
You calculate how large a battery you need to produce the power you need, taking into account sunless days.
So really, the sizing of your system should start with the load. That tells you how much battery storage you need.
Then, knowing that, you can calculate how much panel you need to recharge the battery fully before the next period of use. But even if you use the power for only one day and then give it six days to recharge, the first condition puts a minimum on the amount of panel you need.
For a "typical" 12 volt installation, which never matches anyone's real use case, the load calculation often happens to come out with a number close to the one from the watt to AH "rule". But that is just coincidence. You need to do the real math for your planned use instead.

Regarding the difference between 20 and 100 hour capacity: A battery with high internal resistance will be less able to deliver its full stored energy at high currents, so the difference will be greater. Things like the geometry of the plates, the relative amount of electrolyte and the freedom of the electrolyte to move all affect this. The numbers in Peukert's law for a battery are an experimentally derived figure that describes the actual results based on all of these factors and more.

Sorry guys,
You know, I should have reference linked my other thread ( http://www.solarpaneltalk.com/showth...t-it-backwards ) for you guys just to get the back story ... but basically I know I'm going about it back-a$$wards.

The panels and charge controller were bought long ago. Funds are limited and hardware has already been purchased over the past couple of years. It's too late this time around to calculate my needs first and then build my system accordingly. Essentially, where that last thread ended off was that I'm looking for the largest Ah battery bank I can realistically run with this amount of current being generated from the panels. Once I have that, the usage of the system will be adjusted accordingly.

Because it is a 12V system, the magic number we ended up at in the last thread was 260Ah based on the 12v rule of thumb (1ah per watt). So I set out looking to purchase batteries that would get me in that range. Where I got hung-up on was the Ah ratings of batteries based on 100 or 20 Ah and it stumped me again. Which is what generated this thread.

So I'm gonna look for 6V batteries now to wire in series. And based on what I've kind of learned here is that I'm going for somewhere close to 260Ah based on my dual 12V 130Watt panels. And I'm going to use the battery Ah rating based on a 20Ah 'cycle'??? <--- correct terminology???

So am I going in the right direction here?

g33k

[Naptown]

I would look for 2 4 or 6V batteries in the AH range you need.
My personal favorites although more expensive are AGM batteries.
I like these or a number of reasons.
1- these will take a much higher charge rate than FLA or Gel. Which works well in areas that do not get consistent sun or very little insloation for short periods of time ( Think Seattle or England)
2- Since they are sealed there is very little monthly maintenance.
3- Although not needed in many cases they will perform better in very cold environments.

[Mike90250]

The best "bang for your buck" would be a pair of 6v golf cart batteries, wired in series.

They are about 200ah at the 20h rate, but it will be a lot mor trouble & cost to locate 260ah gear.

[urbang33k]

I would look for 2 4 or 6V batteries in the AH range you need.
My personal favorites although more expensive are AGM batteries.
I like these or a number of reasons.
1- these will take a much higher charge rate than FLA or Gel. Which works well in areas that do not get consistent sun or very little insloation for short periods of time ( Think Seattle or England)
2- Since they are sealed there is very little monthly maintenance.
3- Although not needed in many cases they will perform better in very cold environments.

Gotcha... and then finally, since I'm likely now going with a series wired bank, that sparks a new question just to be perfectly clear.

'The old plan 12vs in parallel' I was going to buy 3 x 90 Ah 12V batteries for a total of 270 Ah

since I'll be wiring in series...

Would I have to buy 2 x 260 Ah 6V batteries. Or 3 x 260 Ah 4V batteries? (just as examples) As I understand it, wiring in series means you dont add the Ah rating together like you do in parallel correct?

[urbang33k]

The best "bang for your buck" would be a pair of 6v golf cart batteries, wired in series.

They are about 200ah at the 20h rate, but it will be a lot more trouble & cost to locate 260ah gear.

hahahahaha that's exactly what you suggested a week ago in my other thread and I'm starting to see the logic in it Mike.

[urbang33k]

The best "bang for your buck" would be a pair of 6v golf cart batteries, wired in series.

They are about 200ah at the 20h rate, but it will be a lot mor trouble & cost to locate 260ah gear.

what ya think about these?

http://www.solartrader.ca/solartrader_ca_-_us_battery_260_ah_agm_-_maintenance_free_en_917prod.html