Battery Amp Hours

[mentalfee]

Question:

Lets assume I have a 200 amp hour rated AGM battery. Is the amp hour rating for complete discharge? If so, and it is recommended that this battery only be discharged to a level of 50%, do I really ony have 100 Ah available in the bank?

Or, is the Ah rating based on 50% discharge recomendations and I do have 200Ah available for my budget?

Thanks

[pvanv1]

The AH rating is analogous to the gas tank in a car. Amps to empty. But in the case of a battery, you don't want to take it all the way to empty.

[celenoglu]

You have 100 Ah available for using, although you can bypass that limit to some extent.

[Zanshin]

And recall that you will only get those 200 rated amp-hours if you discharge the battery slowly (the batteries are usually measured with a 20-hour discharge rate). If you use more than 10 amp-hours from your 200 amp-hour battery then you will get less power out as well. To take the gas-tank analogy further, the amp-hour draw is analogous to your fuel efficiency or MPG.

[btrayfors]

The AH rating generally used for deep-cycle batteries is based on the "20-hour rate", i.e., the number of AH which the fully-charged battery is capable of delivering over a 20-hour period, with the load adjusted so at the end of that period the battery is essentially "flat"....exhausted.

I believe that 10.5VDC is the voltage normally used for that end point.

Other rates are sometimes used as well, and "your amperage will vary". A 200AH battery at the 20-hour rate might be able to deliver 220AH at a lesser rate of discharge (smaller load over a longer time), or with a heavier load (over a shorter time) it would deliver fewer than 200AH.

Then there's the CCA and CA ratings...usually used for starting batteries. And the MCA (marine cranking amps) rate.

It's enough to make a fella's head swim :-)

Bill

[sailingdog]

Yes, the rating is for the battery to be 100% discharged, if you're using 200 amp-hours at the same rate as the battery specifications. For example, many batteries have a 20-hour rating... in the case of a 200-amp-hour battery, that means if you have a 10 amp load on the battery, you will get 200 amp-hours out of it an it will be fully discharged in 20 hours. However, if you were to put a 40 amp load on it, it would not last five hours. The five hour rating would be considerably lower. This is due to the Peukert factor. Likewise, if you put a 5 amp load on it, it would probably give you more than 200 amp hours.

The larger your battery bank, the more efficient it will be, due to the Peukert factor. For instance, if you have four of these batteries. You can set them up as four 200-amp-hour house banks, two 400 amp-hour house banks, or one 800 amp-hour house bank.

Given the same load, the one 800-amp-hour house bank would actually give you more use time, since the load on it is much smaller proportionally to the size of the bank. A 10 amp load on a 200 amp-hour bank is 5% of bank capacity, on a 400 amp-hour bank, it is 2.5% of bank capacity, and on an 800-amp-hour bank, it is only a 1.25% of bank capacity.

Of course, there are diminishing returns to this at some point, since batteries also self-discharge, especially wet cells... and having too big a bank means that the self-discharge will cut in to usable amp-hours before you get to use them.

Space and your ability to charge the batteries is also a major consideration. You obviously need to have the space for the batteries you are using. You also need to be able to charge the batteries after using them.

Xantrex and a lot of the long-term cruising literature has the 35% rule, where the usable capacity on a bank is measured at 35% rather than 50%. The reason for this is that charging a lead-acid battery to 85% of capacity is relatively easy to do, but going from the 85% point, where the current acceptance rate drops dramatically, to the 100% point is often not worth doing every time, since it takes far longer than restoring the first 35% used. Granted, batteries really should be charged to 100% but it doesn't have to be every time you use them, so this is a workable compromise between getting use out of the batteries and saving wear and tear on the boat's engine—which is often the primary battery charging source.

Originally Posted by mentalfee

Question:

Lets assume I have a 200 amp hour rated AGM battery. Is the amp hour rating for complete discharge? If so, and it is recommended that this battery only be discharged to a level of 50%, do I really ony have 100 Ah available in the bank?

Or, is the Ah rating based on 50% discharge recomendations and I do have 200Ah available for my budget?

Thanks

[mentalfee]

Thanks for the details everyone. ?The info helps with the budgeting.

Fair winds.