Dave Plowman (News) wrote:
In article ,
The Natural Philosopher wrote:
Anyone know what the efficiency is likely to be of going
mains--battery charger--battery--inverter--mains?


mm..probably around 90% each way. Say 81%

I'd be amazed if it was anything like that high. Charging Ni-Cads at a
1/10th capacity constant current takes 14 hours. More difficult to work
out with a lead acid as the charge current varies with state but my
reasonably new 8 amp (says 11) charger won't *fully* charge a 75 amp/hour
battery overnight - takes about a day. A good inverter is about 90% though.
I'd guess at more like 60%.

Thats why Nicads are not ideal: Lead acid or Lithium polymer are far
better in the charge cycle. You CAN fully charge Nicads in about 6
minutes..BTW. At even worse efficiency.

The inefficiency of the charge cycle is related to how much the voltage
needs to rise above the off load voltage..with a lead acid its never
more than about 10%.






In other words, how many kWhs need to go into the charger to get 1kWh
out of the inverter?

As long as the overall efficiency is better than about 50%, you
*could* be better off by buying all your electricity at the off-peak
rate and storing it in batteries - but you'd have a hell of a capital
investment (and probably maintenance/replacement costs) to contend
with.

Well I reckoned about 5 grand..batteries SHOULD do what? 10 years? Hmm.
Not as attractive as I thought ;-)

Caravan types would know how long a leisure type battery lasts - I doubt
it averages at 10 years.


I know, but here we are not talking massively deep discharges or
massively high discharge currents, nor are we talking extremes of
temperature. In a house, these things would be running at around a 20-40
hours discharge rate, and about a 1/10th capacity charge rate (i.e. a
110 AH battery charged at 11A or so.

I feel fairly certain that batteries can be optimised for various
duties..car batteries are for short duration high peak output. Leisure
batteries for lower output but deeper discharge. Here we are talking
about optimizing for cycle life. I think 10 years is not unrealistic.
Certainly few car batteries will not - if not allowed to go totally flat
- fail after 5 years.