State of charge charts

[MtB]

It is widely believed and commonly stated on here that when the rested voltage of a lead acid battery is 12.7, the battery is fully charged. I'm not convinced about this since my recent experience with new batteries. I bought two new cheapo 110ah Hankook leisure batteries from Battery Megastore the other day. On arrival I measured the resting voltage on my (checked against a calibrated source) DVM and got precisely 12.7v. Fine, I thought. 100% SoC as expected for a new fully charged battery after 24 hours of resting. 

 

So I bolted them into the boat and fired the engine up, and to my surprise the charge current was 16A at 14.80v. Ok it will probably subside quickly as they are fully charged...

 

But no! One hour later the charge current (still at 14.80v) was still 10A but it was 8pm so I turned it off. Given that my alternator put approx 12AH into those batteries during that hour charging I don't think they can have been at more than about 95% SoC for their 12.7v terminal voltage, and possibly as low as 90%, given the slow rate at which the tail current was tending towards 1%.

 

 

[WotEver]

12.7V is only a guide. That’s why it’s always advised to either take a Relative Density Reading or measure tail current. A higher amount of calcium in the plate composition will raise the fully charged voltage for instance, as will raised temperatures (and the converse). 

[Sea Dog]

24 minutes ago, Mike the Boilerman said:

It is widely believed and commonly stated on here that when the rested voltage of a lead acid battery is 12.7, the battery is fully charged. I'm not convinced about this since my recent experience with new batteries.

Interesting observation. Some charts say 12.6 too. I'd not think it's surface charge either, as they'll have been off charge for a while. Those charts are a rough guide though and temperature also makes a difference. I think it just shows that we need to consider more than one source when we try to decide our battery state and, in particular, when to stop charging.  Although I refer to other readings by pressing the button, I leave my display on the Victron BMV set to Amps when charging and Volts when discharging - I think volts are a better measure of discharge than charge.. 

Edited October 4, 2018 by Sea Dog
I agree with WotEver's post above which crossed in the post whilst I was typing.

[MtB]

 

I wondered if it was something to do with my alternator poking out 14.8v instead of 14.4v...

 

 

[cuthound]

28 minutes ago, WotEver said:

12.7V is only a guide. That’s why it’s always advised to either take a Relative Density Reading or measure tail current. A higher amount of calcium in the plate composition will raise the fully charged voltage for instance, as will raised temperatures (and the converse). 

 

Indeed.

 

When antimony was commonly used to alloy the plates with, 2.6 volts per cell at 15°C was considered fully charged,  however monitoring tail current until it has remained stady for 90 minutes is probsbly the easiest way to monitor charge.

[OldGoat]

Hmm -

battery chemistry used to be simple (to understand). Nowadays there's so many tweaks that manufacturers apply to make a product do what they think the customer needs (not wants).

The vast number of sales is for automotive use - and that's what you get when you buy a low cost unit. low dost doesn't necessarily low quality - they're less expensive because of the savings in volume manufacture and distribution.

A car battery is not suitable for liveaboard boat use, but there are ways that you can adjust your usage to get nearer to what happens in a car -

Lots of batteries (not two 110 Ah budget units) for a start...