Someone kindly sent me a schematic.
Update: Charger goes Bang
Been working fine every day at 14A for years. Today, turned it on,
got a new buzzing out of it, then about a minute later there was a
bang, all the front panel lights went out, and the Bad Smell emerged.
Update: Everything Works
Everything has been working fine. I've added an emergency stop switch
which disconnects the batteries from the inverter, through a 100A DC
solid state relay. The switch is screwed to my desk, in one corner.
Obviously I have no kids.
Update: Earthing Review
Thanks to Martin Merewether for prompting another review of the safety
earth design on this configuration. Martin wrote:
Both amplifier earth and computer earth should be connected to power
supply earth. If you have 100 ohms between the two, you have an
earthing problem of one (or both) devices. I suggest you find out
which and fix it. The earth strap may take fault current, and may
(inappropriately) shunt fault current to the other device, raising the
chassis potential above ground, with safety implications.
The 100 ohms reading was bogus, because current was flowing at the
time of measurement and the meter is not designed for taking
resistance measurements in-circuit. But the hum and voltage
difference remains significant to the discussion.
The safety earth follows this path:
The safety earth for the amplifier departs from this path at the
switched power board. So it is how the earth passes from the power
board to the computer that has to be reviewed.
- earth stake,
- building fuse box,
- building cable,
- building general purpose outlet,
- extension lead,
- switched power board,
- IEC lead,
- charger IEC input connector,
- negative terminal of charger output,
- negative side of battery bank,
- earth lug on inverter,
- output socket on inverter,
The way in which the charger passes the earth to the output negative
terminal is in doubt. It may need to be more explicit. There is no
specification for it to pass the earth, yet I've always known that the
negative terminal to be at earth potential. There is no external
chassis earth lug. With the charger off and disconnected, the DMM
reads 0.2 ohms between the chassis and the negative output terminal,
but the reading took a little while to settle. The DMM also reads
battery voltage between the positive output terminal and chassis.
The next thing to do is to open up the charger and find out how the
earth is passed.
Update: Earth Loop Hum
So, the computer has a sound card, and the output is connected to a
Silicon Chip Studio Twin 50 Amplifier, thence to a pair of quite large
speakers. The amplifier is powered from the same supply as the
charger, not from the inverter. Ever since making this Online UPS,
there has been a persistent 50Hz hum when the amplifier is on.
Using a Digital Multimeter, measuring the AC voltage between the earth
lug of the amplifier and the chassis of the computer ... 0.005V.
Measuring the resistance with power applied to the computer and the
charger ... positive 100 ohms, or negative 100 ohms, depending on how
the probes are attached.
Adding an earth strap fixes the hum. But will that earth strap ever
have to take any fault current?
Two more batteries were added to the bank. These are new VRLA
batteries normally used for traction supply in electric scooters.
They are 40AH each. That brings the bank capacity to 170AH, and a run
time to depletion of about 12 hours.
The charging current is now likely to trigger the over-current
protection feature of the power supply.
It was important to use sealed batteries, since this is meant to be an
office environment. I also had to be able to lift the things. And
they had to be available locally. There are far better batteries to
use, but the intersection of these requirements is a small set.
A few weeks ago, Quozl lost his Matrix 5000 UPS to lightning. So he's
used the bank of batteries with an inverter and charger, to form an
online uninterruptible power supply.
- POWERTECH 500W pure sine wave inverter, 12V DC to 230V AC 50Hz,
- POWERTECH 40A DC regulated power supply, set to 13.8V DC,
- ARLEC 12V 4A charger, for initial startup,
- 5 x 18AH 12V valve regulated lead acid (VRLA) batteries in parallel,
- design loading 288W at 85% efficiency yields 28A draw, so using 8AWG cable which has an enclosed rating of 45A,
- no fuse per battery, so that charge is balanced, (fuses add variation in series resistance to battery),
- earth on negative side of battery bank, charger has earthed negative,
The charger has a voltage and current display.
The battery and output voltage is also checked with a Fluke 10
An energy meter kit is used on the 230V AC output of the inverter.
(Silicon Chip, Dick Smith Electronics part number K 7217)
Computer and LCD Panel
- 13A draw on charger display,
- 168W draw on energy meter,
- 9.7A draw on charger display at 13.7V,
- 123W draw on energy meter,
- therefore 9.27 hours run time to total depletion,
1. Wrong charger for the job
Starting up each morning involves turning on the float charger until
the bank voltage achieves 13.8V, then turning on the regulated
Tested turning off charger for a few seconds, when turning it back on
the display momentarily showed 54A, may cause protection feature to
trigger on charger, may need a series resistance of some sort, no idea
what will happen if it is off for longer while not monitored.
Fan started after 35 minutes with room air conditioning set to 22C,
and ran for a couple of minutes. Mounting the inverter vertically or
adding a low speed fan may alleviate.
3. Unknown efficiency
Casper in ##electronics asks whether efficiency has been checked, so
this will be something to do. The energy meter was moved to the 240V
supply of the charger, and showed 197W. That's an overall efficiency
Calculation based on charge displayed voltage and current suggests
roughly 93% efficiency of the inverter at 123W, which is well within
the specified 85% efficiency at 500W.
| up |