On Mon, 20 Nov 2017 22:53:59 +0000, newshound wrote:

====snip====


Thanks for thoughtful comments. Didn't get to check the voltage yet (mad
day today) but I will do ASAP. I agree, I think these "caravan/boat"
CFLs cope very well with low voltage (which is probably part of the
design spec). One of the huge advantages from moving to "12V" lights run
directly from battery instead of what I used to do, using a cheap
inverter with mains CFLs. Great suggestion from someone on this NG which
had not occurred to me.

As I said before, this isn't a smart charger, just an old fashioned
transformer/rectifier.

Which is all fine and dandy for car batteries and wet cell deep
discharge leisure/marine batteries but deadly when used to charge SLAs
unmonitored.

SLAs, particularly the Gel type (AGM types are a little more tolerant)
don't take kindly to receiving a gassing charge, hence the precisely
defined constant 13.8v charging voltage specified for 12v SLAs to
preclude such gassing charges which will create fixated bubbles of
hydrogen and oxygen in the Gelled electrolyte or within the Absorbant
Glass Matting used to fix the liquid electrolyte in AGM cells. These
gasses are known to behave like insulators just like every other gas at
NTP so such trapped bubbles increase cell resistance and reduce the
effective active area of the plates.

If you're going to use SLAs, it's essential that you use a proper
battery charger or a purpose designed solar panel charging controller to
protect them from overvoltage charging.


One of the reasons I am persisting with "old, knackered" batteries from
diesel cars is that I have now had two specially purchased sealed proper
"leisure" batteries fail after something like two or three years in this
location. They don't get particularly well looked after, they are mainly
needed in the winter so don't necessarily get checked regularly in the
summer. They are connected to a couple of PIR floods but in the summer,
you never see these trigger so the batteries can get flattened by the
cats and foxes without it being obvious. I suppose I ought to add a
"battery low" circuit with a warning light, but that would steal current
and in any case ICBA. In the winter, the PIRs normally trigger when you
arrive in the evening, also you are using the non PIR lights so you know
if the batteries need charging.

One of the things to avoid with SLI batteries is a regime of deep
discharge between each recharging cycle, especially if the battery is
likely to spend a lot of its time in a partially charged state since this
aggravates the sulphation problem on the lowest capacity cell which
accelerates the normal reduction of capacity with each discharge/recharge
cycle. you need to limit the DoD to no more than 50%, preferably 30% if
you want to avoid premature failure under such a regime.

Relying on the brightness of your 12v CFLs won't alert you to the lowest
capacity cell becoming fully discharged in time for you to remedy the
situation with a complete recharge to get all the cells gassing to make
sure that all are brought up to their maximum state of charge to overcome
any imbalances due to variations in the self discharge characteristics of
each individual cell.

Even if all 6 cells in a car battery start off with identical capacities
and self discharge characteristics, it doesn't take too many charge/
discharge cycles for differences to build up which continue to
accelerate, even when given a balancing gassing charge to eliminate one
of the causes for the acceleration of the weakest cell's deterioration
rate. There's a lot to be said for designing equipment so it can be
powered from a single cell "battery" rather than an actual battery of
series connected cells to make up a higher voltage.

If you're planning on a monthly routine recharging cycle, then choose a
capacity that represents at least three month's worth of autonomy to
ensure you don't burn through the limited number of charge/discharge
cycles prematurely. AFAICR, the SLA and deep discharge leisure/marine
batteries can tolerate a 70% DoD without harm provided they're not left
in this state for more than a few days at a time.

It's a relatively sheltered location, otherwise I would be tempted to
put up a small windmill charger, which would probably sort everything
out with leisure batteries.

This can work if you use a suitable charge controller designed to accept
windmill driven PM alternator and/or solar panel inputs which can handle
the 'wild voltages' of a windmill driven PM alternator and match the
varying impedance of a solar panel source to maximise the power transfer
to the battery (as well as limiting the maximum charging voltage to avoid
cooking the battery when you have a surplus of either source of 'Free
Power').

All rechargeable batteries need babying to get the best life out of
them, even the more robust NiFe cells can benefit from a sophisticated
battery charger and are essentially immune to being left discharged for
extended periods of time without harm.
https://en.wikipedia.org/wiki/Nickel€“iron_battery

--
Johnny B Good