I have a cheap Solar alarm system with an internal Alarm Panel fed by
a 12v plug in PSU.
Recently I had a power fail and noticed that the panel went dead..
When I checked the internal Lead-Acid battery it was at 4v and was
Took some advice from our 'Battery Expert' at work and learned that a
Lead-Acid battery charges at 14.2 and floats at 13.8 volts.
So I ordered a new battery, installed it, let it charge for a few days
and then checked the voltage..
I can currently measure 15.1v across the battery, which seems wrong..
But should I worry about it?
99% of people with alarms wouldn't have a 'battery expert' to talk to
and wouldn't even know to worry about this.. It could be a fault on
the panel, but equally, it would be just a cr*p design.
Should I just live with it like this, or should I try and find a way
of bringing the voltage across the battery down a little?
In general a 12v lead-acid accumulator should be kept
between 10.8v and 13.8v. If drained below 10.8v then
excessive sulphate is deposited on the plates. If
charged above 13.8v then excessive gas is produced,
which can result in a significant loss of electrolyte.
In the RS Catalogue the charging information for lead
acid cells is 2.3v/cell float, and 2.4v/cell cyclic.
That's equivalent to 13.8v and 14.4v for a 12v battery.
The RS two-stage chargers generally do a bulk charge
aiming towards 14.4v then, as the charging current falls
back, switch to a 13.8v permanent maintenance charge.
So 15.1v does seem a little high. It could be due to
overcharging, or it could be due to an accumulator with
a higher-than-normal internal series resistance.
My meter is very accurate so its not that.. I have spoke to the
manufacturer today and they have advised that they don't even
incorporate any kind of charging circuitry in the panel.. Its simply
charges across the +v and -v supply to the panel, so this does sound
like a crap design..
In my case, perhaps the PSU is providing slightly too much voltage. I
am guessing that a regulated may not be the solution, as that will be
regulated to 12volts which will be too low for charging.
I will do some checks on the PSU later and see exactly how much
voltage its kicking out and will then try a different PSU to see what
A 12v output would not even maintain the battery.
Just one thing.... There is not much point to measuring the voltage of
the PSU, unless it is under normal load conditions. Off load it will be
likely a volt or two high.
Something which might prove useful.... You can lower the output voltage
of a DC power supply, by inserting diodes in series. Each additional
diode will lower the voltage by about 0.65v. So your 15.1v would drop
to 14.4v with one diode in series, or 13.8v with two extra diodes in
Assuming the PSU is rated at less than 1amp output, then 1N001 diodes
would be adequate.
I was halfway through posting that same idea, when it came upon me all
too embarassingly that if you just stick a couple of diodes in the wire
to the battery, it'll reduce the charge voltage just nicely, but stop
any current flowing back *out* of the battery when the PSU drops out!
A moment's further reflection suggests one way out of that hole is to
put another diode across the two our poster probably wants (for dropping
to 13.8/13.9V from the 15.1) "pointing" the other way, allowing the
backup current to flow when the panel needs it - at the expense of
dropping the 0.6V or so in that case (unless you use a germanium diode
for a smaller drop of around 0.1V).
HTH - Stefek
It can't simply be wired across the PS if you're seeing 15.1 volts, since
a lead acid battery has a low internal resistance and would draw a *lot*
of current when charging. There must be at least a series resistance to
limit the maximum current to something the PS can manage - probably less
than one amp.
*I am a nobody, and nobody is perfect; therefore I am perfect*
Dave Plowman firstname.lastname@example.org London SW 12
Lead Acid accumulators have been around for a long time,
and it is very well-documented that the permanent charge
voltage should not exceed 13.8V..... above this voltage
gas is produced, resulting in loss of electrolyte and a
drastic shortening of the capacity/life of the accumulator.
If all your facts are correct it could be said that the
manufacturers have (knowingly?) produced and sold goods
that are "not fit for the purpose intended". In which
case you may wish to bring it to the attention of your
local Trading Standards department.
Supporting documentation for your case could be;
a) Any Elec Eng textbook that has a chapter on secondary
cells will remark on the effects of charging a lead acid
battery above 2.3 to 2.35V per cell.
b) The battery mfrs data sheets will specify how a battery
is to be charged, including a figure for the maximum
c) A long term monitoring of the battery voltage.
Note; The gasses produced during overcharging are hydrogen
and oxygen. So a lead acid accumulator should never be
overcharged when in an enclosed and/or unventilated space.
Deliberate or unconsidered overcharging of a battery-in-a-box
would seem to be a foolish thing for a mfr to do.
I'd check to see if my meter was reading properly first. The meter may just
be showing the 14.4 volt as 15 ? odds because your meter has a +- variation
on its scale, so this needs to be checked first before you can actually say
that the voltage is to high. This is especially because the voltage you're
reading and the actual voltage that the battery needs on its float charge
cycle are so close to each other.
Yes you should....
Assuming your test meter is accurate, then your new battery can not be
expected to survive for long at that charge voltage. The usual
recommended way to recharge such a battery is at 14.2v until fully
charged, then to fall back to around 13.8v to maintain it.
The only solution I can think of is something to provide a load across
the battery to limit the voltage rise. If you have an electronically
minded mate, talk to him about putting a zenor diode and a power
transistor across the battery.
Thanks all for your help.. I am going to build a regulator, which will
reduce the amount of voltage that I am putting into the pannel.
The regulator I have in mind, will have an adjustment so that I can
vary the voltage.. Then I can 'tune' it to give me 13.8v at the
I dont' want to do anything on the battery input/output, as whilst it
will help the battery charge, it could have implications if/when the
power fails and the battery is used to power the panel.
I think that the best way to fix this would be restrict the voltage
where it comes into the panel.
Many thanks for your advise.. I think that I will remove the battery
for now, so that I don't cook it any more than it already has for the
past few days!
It's not a trivial circuit to design as the output voltage required to
'float' the battery (i.e. not overcharge if continuously connected)
varies with temperature so you have to temperature compensate the
You also probably want to charge (as opposed to maintain) the battery
with a slightly higher voltage to ensure that it gets fully charged
and recharges reasonably quickly.
I recall one of my text books in the dim and dark past had a diagram of a
monitoring battery charger which I built and then found the circuit diagram
had an important error in it so the bloody thing didn't work. I sussed out
what the circuit should have been and corrected it, then it gave sterling
service for many years looking after the battery on a standby diesel
generator. Prior to that time the battery was steadily bubbling away and
drying out when it got forgotten.
I incorporated a charge ampmeter and it was interesting to watch the battery
being charged up after use until the float voltage reached design level then
the charge current dropped to zero with occasional kicks on the meter as it
detected voltage fall and popped a bit more charge in to compensate.
I think the whole box of tricks came to about 20 quid in them days including
a cheapo Halford charger which provided the transformer casing, and leads
etc (time not included). I cant recall the book title but I'm pretty sure
the author was Olsen or something like that.
The easiest thing to do would probably be to replace the
existing plug in PSU with a plug in Sealed Lead Acid charger,
and these are not prohibitively expensive.
Only issue is you would always have to keep a battery connected,
as the charger would likely produce strange voltages if you
tried to use it as a power supply with no battery load.
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