Thank you for the respective replies, Colin & Kate. These are most helpful,
as from them, a picture of the situation is now emerging to me, such that
there may be two different issues being talked about in battery-flattening
discussions.
The first issue is that of MX5 batteries going flat while car sits unused.
The second is one of such batteries not recovering when recharged, and hence
being discarded at young ages.
From this, I can clarify that the second issue isn't what I'm trying to dig
into, as that issue doesn't arise with us - if the MX5 batt has flattened
while car unused, then we just charge it up and everything's tickety-boo,
exactly as we've done with our other cars that have sat around unused from
time to time.
It's the first issue above that I'm trying to clarify - i.e. do unused MX5s
flatten their batts more than other unused cars do?
On this, it's been agreed that our MX5's standby current, abt 20mA, is no
greater than that of many other cars, and in any case I don't think 20mA is
a big deal in terms of the flattening issue. So the level of standby
current can be disregarded as a significant factor. And since we use
ordinary batteries in our MX5 (batteries get swapped around amongst our
assorted cars as required from time to time) then the special battery-type
originally fitted to the MX5 can't be a factor either, in our case.
The logical conclusion has to be that (in our case, anyway), the MX5 can't
really be flattening its battery any more than the same battery left in any
other of the unused cars around the place. Any perception we've had that it
does do that would seem to be just our imagination (albeit, imagination
fuelled by the discussions we've read on the 'net!). But the only way to be
sure would be for us to make proper notes on how long the respective cars
sit around unused on each occasion and how their batteries cope with the
non-use. However it's not such a big issue for us now, as these days we've
cut down on the number of batteries we have in the place - we've got a good
few cars but only the MX5 and a couple of others are continuously
registered, so nowadays we only have three batteries. If another car is to
be registered and used for a while, we borrow one of those batts for it.
So these days the MX5, being a sunny-days sports car, is the only one that
sits around unused with a battery in it, and hence the only one we have to
keep an eye on. I know that letting an unused battery self-discharge, and
leaving it like that, will damage it, so from now I plan to recharge the MX5
batt periodically while car is unused. Since it's an ordinary wet batt the
standard simple charger we've always used on all our other batts will be OK.
Although it may be "OK", I'm aware that such a charger won't be IDEAL for
maintaining a battery in storage. The requirements for the long-term
storage-charging of battery are different from those of the simple one-off
recharge, and ideally you need a charger designed for the storage-charge
application (and also tailored for the particular type of battery you are
storing). I've done a bit of Internet research on this and I gather what's
needed is a constant-voltage charger, regulated to a voltage a little below
the gassing-point of the battery concerned. This is a simple thing to make
and it's on my project-list (but so are a hundred other things).
Regards,
Graham H