Originally Posted By: wrcsixeight
Reserve capacity is not the same as Amp hour capacity.
They are related, but a new fully charged 100 amp hour battery can provide 5 amps for 20 hours before voltage falls to 10.5v which is considered fully discharged.
reserve capacity is how many minutes a battery can provide a certain amount of amperage before a certain voltage threshold is reached.
This certain amount of amperage and certain voltage should be standardized for a good comparison, but they could decide to change those factors to make it look a bit better. lower the load and also lower the voltage threshold, and then claim a higher RC.
If the RC test is equal for each battery and one provides more minutes, means it is is less affected by the peukert effect.
The peukert effect basically says the higher the load the less capacity the battery can provide.
So higher RC should mean a better battery, but not necessarily so.
A battery could be designed to do very well in capacity tests, AH and RC, but fail sooner as the lesser resistance means more porous fragile plates more prone to shedding and breakage.
This sums it up pretty well.
Ampere hours for lead acid batteries is quite the ripoff, as it isnt done at any sort of practical rate. It is done in a manner that the battery discharges at an 8 or 20 hour rate, and the amp-hours are calculated.
RC is done at a constant 25 Amp load. It is measured in minutes...
According to Pacific Power, to convert from RC to Ah at the 25A loading rate, multiply by 0.4167.
So the 90RC battery can provide 37.5Ah at 25A discharge rate; the 100RC battery can provide 41.7Ah at the 25A discharge rate.
Why would this be? Well, for one thing, one manufacturer may be giving an Ah capacity at C/8 while the other at C/20. Another could be the end voltage per cell under load to get the Ah rating (RC is standardized, Ah may not be).
Unfortunately there isnt a ton of data on starting batteries.
A good analog may be the C&D high rate VRLA, yes it isnt flooded, but we have to look in general ranges to see what is possible, and only so many good spec sheets are available... Folks that wish to handwave an argument of that type are free to find SLI batteries with full information to share.
https://www.cdtechno.com/pdf/lit/12_1029_0915.pdf
A few things you can take away from this document are as follows:
Some operational data that defines the Ah of the sizes of batteries:
Granted this is taking rhe battery conduction voltage down to 1.67V/cell as opposed to 1.75V per the RC test.
Now lets look at the 150, 210 and 300 ranges:
Using the 24A constant current discharge to 1.75V/cell as a proxy for the 25A reserve capacity assessment, we can then interpolate values:
So, we know the following:
- UPS batteries, though designed for "high rate", are also designed for longer typical discharges than a Starting, Lighting and Ignition (SLI) battery.
- VRLA generally operate better in every way than flooded.
Judging from these data, a 45Ah battery would give you roughly a 90-100 minute RC. But that's a VRLA, that's a new and well-kept battery, and that's assuming minimal losses or impedance increases. It is hard to determine if the 60Ah rating of the one battery is optimistic, or else its impedance is high enough that it polarizes at reasonably low sustained currents (i.e. 25A) that arent observed at a 20hr rate, or, if the 60Ah battery may use the same innards as another higher rated battery, and is just meeting a "minimum" nameplate.
Id suspect that the 45Ah battery is less honest; but IMO, Id buy the heavier one, even if it was rated at 45Ah... Since the only actually meaningful measurement you can do with batteries is get their mass... which equates to quantity of reagents.
