Reserve Capacity Question (Battery)

sjturbo wrote:
I am trying to compare battery's and they do not make it easy. Battery states:
Reserve Capacity as 125 minutes at 75A. I need to know what it is at 25A? Can anyone help? Please.

Here are the specs for a T-105 as an example. scroll down to the "electrical specs" for 75 and 25 etc to give you an idea.
(even better would be to look up the specs for your battery)

https://www.trojanbattery.com/pdf/datasheets/T105_Trojan_Data_Sheets.pdf

well IF your numbers are correct
thats 375 minutes at 25 amps, thats 6hrs 15 minutes or approx 150 amphrs

must be a BIG battery over 100 lbs weight

my 135 AmpHr telco AGM is 102 lbs

sjturbo wrote:
I am trying to compare battery's and they do not make it easy. Battery states:
Reserve Capacity as 125 minutes at 75A. I need to know what it is at 25A?

Why? What you really want to know with any deep cycle battery is what it's amp hour rating is at the 20HR rate, the most common reference for comparing deep cycle battery capacity. Any battery for which this value is not listed isn't a deep cycle battery.

Reserve capacity is figuring out how long a battery can last powering a car with fuel pump and ignition load (no lights A/C or other loads) until the engine utterly stops dead in the road.

Sucking down a battery to ten volts is hard on it especially if it's a thin plate engine starting battery. There are legitimate reasons to want a high R/C like extending runtime in very rural areas. But if that is your concern use multiple batteries.

This value can assist a buyer who wants to judge if CCA is impinging on ampere hour rating. But by itself it is almost worthless if extremely cold cranking amperage is vital or low amperage discharge capacity is concerned.

For cycling, ampere hours / weight is a lot more valid. As is the name of the manufacturer. Cold cranking amperage is the vital value when a battery attempts to crank over something tough like a diesel.

An intriguing reference is to compare a battery's 20 hour rate to it's 100 hour rate. Smaller batteries end up in fractions of an ampere. You'll never see a smaller battery's 100 hour rate. It's like sneaking up behind it and yanking down it's drawers. Or, maybe inquiring about plate thickness.

It can get confusing. The last time I was battery shopping I resorted to comparing by weight, Lbs/$, or price per pound if you will. The Wal-Mart 27DC won for what I needed.

It is actually a more complex issue (going from higher rate of discharge to lower).

Might want to look at this: https://en.wikipedia.org/wiki/Peukert%27s_law

So, without getting into a lot of math, I would ask the battery manufacturer OR get the "standard" 20 hour spec.

Well as I suspected this is way more complex than I was hoping. So boiling it down my real question is which battery: Crown CR235, 235AH, Reserve Cap. 125min at 75A OR Interstate 2300, 242AH, Reserve Cap. 140min at 75A? I run 4 series/parallel. I think the Interstate is about the same price as the Crown. I really appreciate everyone's input.

you had all the info you needed
crown 235 ampHrs (normally listed at the 20hr rate)
meaning 11.75 amps for 20 hrs, which is a higher rating than 'reserve'
even calculated at 25 amps
6v series-parallel 470ampHrs
are you trying to figure the run time for a 50amp load ?

sjturbo wrote:
Well as I suspected this is way more complex than I was hoping. So boiling it down my real question is which battery: Crown CR235, 235AH, Reserve Cap. 125min at 75A OR Interstate 2300, 242AH, Reserve Cap. 140min at 75A? I run 4 series/parallel. I think the Interstate is about the same price as the Crown. I really appreciate everyone's input.

I'm not sure I see your confusion here. 242 Ah is more capacity than 235 Ah (though not a lot more), and likewise 140 minutes reserve capacity is greater than 125 minutes, both at 75A. The Interstate is rated as having a bit higher capacity.

Or are you asking which company makes a better battery? That's a rabbit hole of a discussion.

Sjturbo, - for comparison shopping and for somebody without much technical knowledge the only thing important is 20H amper-hour rating. If it's not there, then it's not a battery suitable for offgrid use.

For camping with hook-ups only, any cheap small battery will do.