deep cycle/dual purpose battery?

workhorse,

starter batteries are designed to deliver enough amps down to minus 18 degrees.
Because any battery will drop off in capacity as well as internal resistance at lower temperatures, they're made with a high active surface area in an attempt to offer enough leeway to the negative temperature affects.
In other words, they have a higher number of relatively thin plates (electrodes) and just as many gaps between them filled with electrolyte.
Deep cycle batteries are the opposite. Because the plates are thicker, and their number is smaller, there are less spaces. You could say deep cycle batteries have a higher lead 'fill factor'. That's why they're heavier than starter batteries.
And because lead is the most expensive ingredient in a lead acid battery, they're also dearer.
Because cranking only requires about 250A max, a sizeable deep cycle battery offers low enough internal resistance to supply this kind of current - under normal temperatures. But at -18 degrees, the same battery will struggle while the starter battery with the high plate count will just keep cranking (if it's fully charged).
So in Australia deep cycle batteries can be used for cranking most of the time.
One reason why this isn't really recommended is because of the higher than ambient under-bonnet-temperatures, and you can't afford to lose electrolyte in a sealed lead acid battery.
And the other reason is, that if you draw a lot of Ah before starting, your typical deep cycle battery will take tens of hours to get fully recharged with the low 13.8V alternator voltage.
So you want a good charging system for any deep cycle battery on bord (or recharge it regularly by a good mains charger).
The charging system needs to output 14.7V of boost voltage for quick recharging. But even then, it may still take longer than 10 hours to fully charge a 80% discharged deep cycle battery.
Starter batteries never get discharged more than to 90% SOC normally, so this is a non issue because they're operated in float mode basically. If you tried to discharge a starter battery more than this on a regular basis, the recharging times will increase dramatically, and at the same time the battery will suffer from effects like positive grid corrosion (which raises the internal resistance and leads to grid embrittlement). Because you then will operate your starter battery in a partial state of charge for most of the time, the battery will also sulfate up.

If you want excellent cranking and good deep cycle properties combined in one battery you'll have to look at a variation of AGM: pure lead/tin, spiral wound.
There is one for you to check out:
http://www.batteryvalue.com.au/spiralwoundAGM/

For any questions, just use the form on the page, or post back here.

Best regards, Peter

I am using a Fullriver HGL120 (120Ahr AGM) quite successfully as a crank battery in the OKA. It is NOT "under the bonnet", so excessive heat is not an issue. I use Fullriver DC115s for the house.

Cheers,
Peter
OKA196 Motorhome

The real life difference between deep cycle and cranking batteries is not much. A lot of what you read is hype. One battery retailer admitted to me that their deep cycle batteries were the same as their cranking batteries with a different sticker and half the warranty.

Good to see some informed battery knowledge!
Also a recognition of the different construction methods.

Some statements are not incorrect but somewhat interesting - like AGMs gassing less that wet cells. (I usually describe it as not overcharging (over-volting) an AGM because they cannot dissipate gas etc and instead may initiate a thermal runaway, and (hence) also the danger of AGMs with collapsed cells.)

A cranking battery sold as a deep cycle would have a lesser warranty because either can be used as the other - they just won't last as long.

Where dual-battery set-ups have been sought - although it depends on the aim & application - I usually recommend a simple co-charging arrangement with the standard battery and whatever other battery is desired - eg, AGM if not vented; deep-cycle for fridges and drive-in theater audio systems ; crankers for audio SPL competitions and doof ride-through.
I strongly advise against parallel batteries when merely idling (not being charged, not with a reasonable load) even if they are "matched".
In most systems, the difference between a permanent parallel battery installation and one that is paralleled only whilst charging is a mere relay - typically $10 for 60A etc.


Where wet-cells can be used, I was recently impressed by a $180 dual-terminal deep-cycle wet-cell with ~760CCA & 100AH .

And despite the old 70/30 rule - where you shouldn't discharge deep-cyclers/crankers more than 70%/30% - being surpassed by alleged 90% and even 100% dischargeable deep-cycles, the word still seems to be "not more than 50%".

The only other thing I seem to consistently hear is how over-rated certain big name AGM/VRLA batteries are. O for the life of me, I won't mention their nameO. (But both have cheaper rebranded versions.)


Finally - as I just wrote in another thread - despite deep-cycles not being good for cranking, if their capacity is big enough, the cranking current has a lesser impact. (Is it too late to mention a "parallel during cranking" bypass where only deep-cycle batteries are used?)