Converting Reserve Capacity Minutes to Amp Hours

[Pat Anderson]

OK, I need to know the Amp Hour (AH) rating for my house batteries, which are two NAPA Premium RV/Marine Deep Cycle batteries wired in parallel, in order to set up my Victron BVM 700 battery monitor. Each battery is rated at 160 minutes Reserve Capacity. The Victron BMV 700 (picked it up yesterday) needs to be set to the actual capacity of the batteries in AHs.

So I THINK I get there this way: Add the RC of each battery separately to get total RC. 160 + 160 = 320. Divide RC by 2.4. 320 / 2.4 = 133.33 AHs. The Victron defaults to 200 AHs on startup, so I will have to lower the setting to 0133 (four digit display). Agree, or am I missing something here?

The "divide by 2.4" is an alternate method that gives the same results as the technically correct method (copy and paste from eHowTo.com):

Instructions
1
Multiply the reserve capacity by 60 to convert it to seconds. For example, if a battery offers a 100-minute capacity: 100 x 60 = 6,000 seconds.

2
Multiply this length of time by 25, which is the battery's amperage. Example: 6,000 x 25 = 150,000. This is the number of Coulombs of charge in the battery.

3
Divide this answer by 3,600, which is the number of Coulombs in an amp-hour. Example: 150,000 ÷ 3,600 = 41.67. This is the number of amp-hours in the battery.

Tips & Warnings
To convert in a single step, divide the reserve capacity by 2.4.

[TyBoo]

For steps two and three you must wear a tin foil hat to prevent any alien Coulombs from intermingling with resident cerebral Coulombs thus creating contrary Coulombs and contaminating the Coulomb pool. This is especially likely in situations of over thinking the situation. And thats all I know about that.

[thataway]

Some say divide by 2.4 some say divide 2 some say divide by 2 and add 16. The reality is unless that specific battery (batteries have different characteristics, when tested for AH vs RC, as related to discharge curves/vs current, temp etc) has been tested and certified at Amp Hours, it is just going to be a close guess. Also in reality, the battery capacity changes as you use it. You will be conservative setting at 133, but it is probably a little more than that, if you used the divide by2 and add 16.

It really does not make a lot of difference--and with the conservative number, you will be far less likely to damage the battery by deep discharge.

I try and not drop the steady state voltage to less than 12.2 volts. This gives you long life of the battery. If you don't pull more than 75 amps out of the battery between re-charges (66 2/3 if you use the number you have) you should have good use of the battery. I used those same NAPA batteries in a Road Trek RV for over 6 years and they worked very well.

[Pat Anderson]

The Victron shunt is now installed. On to mounting the display and running the cabling!

[Molly Brown]

Mile I'm with you. Bob's response is the same as yours to me, lol.

[Aurelia]

Hi Pat,

Your NAPA batteries are regular lead acid and produced by Deka in the GP27 size so they should be 90AH rated each at the 20a rate so you have yourself 180AH for that new freezer of yours. I am assuming they are the same batteries as their Marine Master line which is a pretty safe assumption in that business. So your 133 value is conservative for sure.

See this spec sheet:

http://cdn.shopify.com/s/files/1/0033/6262/files/dc_series_PDF.pdf?1254611993

[ssobol]

Doesn't it just say this on the battery? Usually in small text someplace because they are not usually marketed by amp hours. IME all the batteries I have seen have this marked on them somewhere.

[thataway]

I looked at my Sam's Duracell, AGM batteries, which are the same as the East Penn (Deka) "Intimidator", and they have the same amp hour and residual capacity as the flooded lead acid batteries, in the same size case--as Greg posted in the link. But the AGM batteries weigh 10# more. (which means 10# more lead. The group 31 in the AGM (and flooded lead acid) are 200 residual cap and 105 amp hours. (Also the group 31 comes with 3/8" post rather then 5/16" post).

[bcassal]

I called my friendly NAPA store and asked them. They got back to me with the rating which was not on the batteries.

Carpy

[jkidd]

In the manual it says to use the 20 amp hour rating. So 2 batteries in parallel should be times 2.

[journey on]

There is a relationship between reserve capacity in minutes and amp hours which might give some insight into how to get a meaningful amp-hr rating from reserve capacity.

First, reserve capacity is defined the number of minutes a fully charged battery at 80 ° F will discharge 25 amps until the battery drops below 10.5 volts. Note the 10.5 volts, that's certainly lower than one would want to go, but that's the definition. And 25 amps output is a tough test for a battery.

So the amp-hrs produced is 25 amps time the minutes/60, right? So using Pat's batteries as a baseline, we get 25 amps*160 min/(60 min/hr)=67 amp-hrs per battery. The 2.4 used above is simply 60/25, so Pat got it right. And Pat has 2 batteries so 67+67=134

A Group 24 flooded battery is rated for 75 amps (West Marine catalog), so though Pat doesn't say, I assume those are Group 24 batteries.

Now, an important consideration is making the battery last. Discharging a lead-acid battery to 10.5 VDC is going to kill it real quick. A good rule of thumb is to use 50% of A-H capacity. Therefore, Pat has 67 amp-hrs available. And he ought to monitor the Vectron monitor so that he doesn't exceed that.

Also, AGM batteries have the acid retained by fiberglass mats and require relief valves to eliminate overpressure from any fast charging.. I would assume that they are heavier than equivalent flooded batteries because of the added equipment, not necessarily from added lead. And the case size is the same, so getting more lead in is problematical.

Boris

[Pat Anderson]

Jody, what manual? My batteries have an RC rating only, no AH rating. I am going with the "conservative" conversion from 160 RC x 2 (320 RC) to 133 AH. Maybe right, maybe wrong, we shall see! Boat is outside right now, with the solar panels pumping out energy all afternoon. Middle of the afternoon, the Victron said we were putting 5 amps into the batteries. All day long, SOC was at 100%. In the dark, no energy being made, no load, the Victron said 99.99% SOC. I will check in the morning and see what we have! Tomorrow I will hook up the ARB 50 quart fridge/freezer, and see that that load does to the batteries. I can hardly wait to get to Powell, where we will have some real sunshine!

[Pat Anderson]

Boris, I agree, we do not want to kill these batteries! So will be watching the Victron really close for a while!. I am thinking though that with the ARB at a max 1.3 amps and the other loads like our LED lights being really low, we should be just fine. The only question is what load Webasto may put on the batteries, but this time of year, we don't need to run, it so that is a question to answer in the future!

[jkidd]

[thataway]