Reserve Capacity to Amp Hours

[FourPoints]

was trying to look up specs on a Die Hard Marine Starting battery I had to determine the capacity in AH, but wasn't listed anywhere. The battery is marked with a Reserve Capacity of 90, so after some digging I found the following formula:

Amp/Hours = (Reserve Capacity / 2) plus 16

So my 90 RC starting battery = 61 AH if used in a house bank

Thought I'd share this as I don't recall seeing this formula posted before.

 

[Maine Sail]

was trying to look up specs on a Die Hard Marine Starting battery I had to determine the capacity in AH, but wasn't listed anywhere. The battery is marked with a Reserve Capacity of 90, so after some digging I found the following formula:

Amp/Hours = (Reserve Capacity / 2) plus 16

So my 90 RC starting battery = 61 AH if used in a house bank

Thought I'd share this as I don't recall seeing this formula posted before.

That formula is rather inaccurate. For grins just apply it to any number of batteries rated in both Ah and RC you find some will be close and others off a bit. I find RC / 2 to be a little more accurate but as far as I know there is no good way to convert and be 100% accurate due to our good buddy Peukert...

 

[FourPoints]

I found that on the University of Hawaii chemistry department website...

http://www.chem.hawaii.edu/uham/bat.html

I understand all of these numbers are for lab conditions and test batteries in perfect condition, and in the real world rairly perform to spec, and that the Peukert effect always skewes the actual result. Since I was just going for a rough idea of what the battery should do in ah, for a baseline comparison between this and other batteries, not factoring in age, I think it's a close enough estimate for what the battery would have done when new, but given the fact that it isn't new anymore, your formula is probably going to be closer to how it actually performs now.

 

[Ron20324]

Thanks, FourPoints. I just installed a Victron but had not programmed it since one of the 3 batteries was a starter.

 

[Anchor Down]

William Tell Splits an Orange w/ an Arrow

FP, you gonna start deep-cycling that starting battery?

 

[FourPoints]

FP, you gonna start deep-cycling that starting battery?

not exactly, but it has been part of my house bank since I bought the boat, and will remain so for the rest of this year, and probably next year as well.

I'm pretty good about normally not drawing the batteries down too far, but it will get a deeper than it should cycle now and then. I wouldn't have bought a starting battery, but since I already had it, I wanted to know the specs. I plan to replace it next year or the following year, depending on how it continues to perform.

Any time I go out for overnight trips I will leave the starting battery disconnected from the house bank (via the 1-2-B switch I installed this spring), but when the boat is sitting at the dock or for quick day trips, it will be connected so the solar charger keeps it topped off.

 

[Maine Sail]

not exactly, but it has been part of my house bank since I bought the boat, and will remain so for the rest of this year, and probably next year as well.

I'm pretty good about normally not drawing the batteries down too far, but it will get a deeper than it should cycle now and then. I wouldn't have bought a starting battery, but since I already had it, I wanted to know the specs. I plan to replace it next year or the following year, depending on how it continues to perform.

Any time I go out for overnight trips I will leave the starting battery disconnected from the house bank (via the 1-2-B switch I installed this spring), but when the boat is sitting at the dock or for quick day trips, it will be connected so the solar charger keeps it topped off.

Combined during charging is fine but it is not good to mix battery types, like thick plate deep cycle & thin plate starting batts, when discharging. I would strongly advise NOT adding in the Ah of the start battery and ONLY monitoring the bank you use as "house". Use the Ah for that bank and wire the shunt off that bank only. Trying to monitor banks you can and will separate will drastically throw off the calculations of the battery monitor. Plug in the house bank Ah and monitor that bank only.

Ideally there should not be a need to even use your "start" bank. Just use the house bank for everything. If observing the max 50% DOD you'll never be left without the ability to re-start your motor unless the batteries are getting long in the tooth.

Here's a couple of examples of the formula's not working well:

T105 = RC-447 / 2 = 223.5 + 16 = 239.5Ah (the real Ah capacity is 225Ah)

Trojan GRP 24 AGM = RC-137 / 2 = 68.5 + 16 = 84.5 Ah (real Ah capacity is 76Ah)

West Marine 4D = RC-290 / 2 = 145 + 16 = 161Ah (real Ah capacity is 135Ah)

West Marine Grp 27 = RC-150 / 2 = 75 + 16 = 91Ah (real Ah capacity is 80Ah)

 

[calpico]

Here's a couple of examples of the formula's not working well:

T105 = RC-447 / 2 = 223.5 + 16 = 239.5Ah (the real Ah capacity is 225Ah)

Trojan GRP 24 AGM = RC-137 / 2 = 68.5 + 16 = 84.5 Ah (real Ah capacity is 76Ah)

West Marine 4D = RC-290 / 2 = 145 + 16 = 161Ah (real Ah capacity is 135Ah)

West Marine Grp 27 = RC-150 / 2 = 75 + 16 = 91Ah (real Ah capacity is 80Ah)

The Capacity_Ah = (ReserveCapacity_Min / 2) + 16 formula is a rough approximation of Peukert's Law that can easily be done without a calculator. It is reasonably accurate only in the range of 100-180 min reserve capacity (essentially the automotive range).

You can use Peukert's Law to get a more accurate result, without the range limitation:
C = I * H * (t/H)^(1/k)
C: rated capacity (Ah)
H: rated discharge time (20h for auto/marine battery Ah ratings)
I: actual discharge current (25A per reserve capacity definition)
k: Peukert constant (1.20-1.22 for auto/marine lead/acid batteries)
t: actual time to discharge battery (in hours, this is the reserve capacity)

Plug in the above numbers and adjust for units:

Capacity_Ah = 1.39 * (ReserveCapacity_Min ^ 0.83)

Using this formula, you should get results that match the manufacturer Ah ratings more closely. This is as accurate as you can get based on the reserve capacity number alone.