# Battery charger for sealed lead acid

Discussion in 'Hobby Electronics' started by Arpit, Oct 8, 2003.

1. ### ArpitGuest

Hi, ive had a powertech 12 volt sealed lead acid battery charger. It
puts out 350 mA. Its a plugpack, and I bought it from Jaycar. It has a
single LED, which flashes slowly (stays on) when charging, and flashes
fast when trickle charging, apparently. I use it for charging a 7 amp
hour battery on my bike for the headlight. THe headlight is a 55 watt
number, so drains it pretty quickly. I looked at the data sheet for
the battery, and it says the standby cahrge voltage is 13.5-13.7
volts, and the cycle charge is 14.5-15 volts or thereabouts. I beleive
I should be charging at the cycle charge rate, since im not leaving
the charger on for months without using the battery- at most a week.
How do I find out what voltage the charger puts out? A trickle
charging battery measures 13.5 volts, open circuit reads 0.5 volts
(it wants a battery), and connected to a capacitor (15000ufd), the
voltage varies from13.5-15 in a fairly regular pattern. Theres a
variable resistor on the circuit board (I cracked it open with my
vice), as well as 4 transistors, 6 diodes, 2 electro caps, and a few
resisors. ANy help/info/advice would be much appreciated as to what
charger ive got, how its charging, what voltage its charging at, and
whether I should get a new one. Thanks

2. ### Terry CollinsGuest

From what I've read, for maximum battery (cycle) life, you should have a
maximum discharge rate of C/10. so your &amp hour is safe to drive a 8.4
Watt (12V) light.

Some people argue that C/5 is acceptable to modern batteries, so that
give you 16.8 watts (Say 20Watt). This is what my wife bicycle lighting
is (20W front + 6 Watt rear) on a 7amp hour battery. since she uses it
for about 1 hour per week, I think it is a reasonable cost effective
arrangement. Lasted over a year so far. Charger is one of the Powertechs
that you mention.

I have massively more lights, including a 50watt + 20watt, but I run the
18 Amphour, which only gets hit for 30 minutes per day (5 days per
week). The Powertech charger works okay. Again for a years use.

My 2c is that you would only need another charger if you are not getting
sufficent recharge between use. Although, with what you are doing, I
would also log starting voltages so you can start to see when the
battery is starting to fail.

3. ### RussGuest

I'm pretty sure it is correct to measure the voltage across the battery with
the charger connected, so the 13.5v trickle charge looks to be correct.
However, 55 watts at 12 volts is about 4.6 amps, which is pretty hefty for a
gel cell - any reduction in life is probably going to be due to that moreso
than charging factors.

Since 7Ah cells are pretty cheap these days, I wouldn't worry prematurely
since you can get a replacement for about \$17-\$20.

I'd be more worried about motorists getting blinded by your 55w lamp -
particularly if it's a 38 degree halogen.

Russ.

4. ### ArpitGuest

Yeah, I was more thinking about time between recharges as opposed to
whole life...
Suffice to say Im visisble

5. ### ArpitGuest

\

What trends should I look out for? bearing in mind the battery sits at
13.5 volts atm,

6. ### Terry CollinsGuest

With the lead acid batteries, they recommend taking them off charge,
leaving them stand for an hour, then reading the voltage across the
terminals. Compare to recommendations.

I've done similar with SLA's with unknown history that have come into my
hands. I give them a few charge/discharege cycles and measure each time.
If they don't keep the charge, then they are cactus. I think it is <
10.5V, then rubbish. Also easy to spot the ones getting dodgy.

So, if you are charging them up overnight, disconnect and commute to
work, measure the voltage on arrival at work and just log it. If you
graph it, then over time, you be able to spot when it is going off.