Emergency Reserve Battery

Well, after all this time, it was bound to happen...

We had a run of partly cloudy days that then ended with a gloomy rainy day.

I didn't notice that the lithium battery was getting very low until the "5 minute warning" police siren sounded at after midnight.  The fridge had just clicked off on the thermostat and we were about 20 minutes away from a forced shut down (the fridge start up current would push the battery down below the CellLog8s alarm set point).

So I switched the fridge and a few other things back to the mains and had a rummage in the "lab" for an old 200W 12V inverter.  This still had some crock clips attached permanently to its inputs (hadn't got round to cannibalising it for the wire, fuse and clips for anything else!).  Then I plugged one of the spare 80W lab power supplies into it and set it for 3A limit at 24.5V.  Just enough to hold the reduced house load up, without trying to charge the lithium bank at all.

I connected the lab power supply to the left over charger inputs on the terminal board between the lithium bank and the house inverter and hoped that it would be enough until dawn.

Despite being a quite big lead acid battery (180Ah).  I was using only one at 12V to power a 24V system, so its effective capacity is only 90Ah at 24V.  And you can only really use about half of that, so about 45Ah at 24V.  Still, that's 15 hours reserve at a 3A load from the house inverter.

Not efficient at all, being triple converted, but it worked for 5 hours of emergency load support before dawn.

With the sun up, I reversed the lab supply, plugging it into the unmetered AC output on the house inverter (so as to not count the kWhs put back into the reserve battery) and charged it back up at a constant 5A.  This way the reserve battery takes priority over the lithium bank.  The lithium cells don't mind being left at low charge for ages (they actually rather like it), but the lead ones need to be fully recharged as soon as possible.

This could be a start of a hybrid battery system, where the lead acid battery is only used infrequently to back up the house battery in an emergency and then recharged as a priority.  Used like that, a lead acid battery will last for many years.

Having three of these lab power supplies available, I could support a load of up to 240W for a short time (up to 5 hours per 180Ah 12V battery).  Or I could dig out the 1kW 24V inverter and use both of the 180Ah batteries at the same time.  But I rather like the ermm... "compact" arrangement of a single battery and a tiny inverter that can just be moved around without having to mess about with bolting batteries together.

In theory, the SmartGauge alarm relay could turn the emergency reserve chargers on, but the lab supplies have a safety feature that means they default to the outputs being off when they first power up.  So someone would still have to be there to press the "go" button on them.

Mercifully, this new Ritar lead acid battery that I used is behaving better than the one I sent back to the supplier a couple of weeks ago. 

After much e-mail to-ing and fro-ing, and measuring and testing, the supplier gave in and agreed that the battery that was gurgling and farting under only moderate charge and making a terrible noxious stink was faulty and so swapped it for a new one.

I didn't have the heart to tell him that I wasn't planning on using either battery any more, as I'd gotten a lithium battery bank in the meantime :D.

The "5 Minute Warning" Alarm

The CellLog8s LVD is fine and works well at protecting the battery bank from over discharge but it doesn't give you any warning that the lights are about to go out.

So I started thinking about some kind of pre-alarm that would give me a few minutes warning (under heavy load) that the battery was nearly depleted.  I could then turn off some big loads and buy some time.

The old SmartGauge Voltmeter has a programmable relay in it too.  But it only works on pack Voltage.  I've been using it as an obvious visual reminder of pack Voltage (it's no good at reading SoC for non lead acid batteries).

Then I started looking around the house for something to use as a buzzer or alarm sounder that the relay could activate so that you'd get the message...

I found a small toy sound effect thingy that you press the top and it makes a cool police car sound.  I actually found another one that my wife had that made steam train noises and in fact a whole load of similar sound chip enabled things, like a Dr Who Darlek bottle opener that says "EXTERMINATE!" when you close a circuit (with the bottle top) and a Christmas card that plays George Michael's "Last Christmas"...

But I decided to go with the police car :D

Taking it to bits was very easy and then all I had to do was solder wires on to the existing switch contact and run these out to the alarm relay on the SmartGauge and program the chosen low Voltage alarm.

The toy still uses the same two 1.5V button cells to make the noise.  The SmartGauge does not provide any power to the relay contacts so external power for the alarm or whatever is needed.  If the batteries in the alarm go dead, it does not affect the safety of the battery bank as this alarm is just for information.

When the relay contacts close, the police siren only goes off once for a few seconds and then stops.  This is a good thing!  Saves the batteries in the alarm and prevents bricks being thrown at the thing for sounding too much once the message has gotten through to whoever is within ear-shot of it :D