How to charge 12V SLA backup battery from 12V battery bank?

[Lomax]

Hi all,

I would like to add a backup battery to the on-board computer system on my boat; just a small SLA alarm system type battery which can keep the computer and 3G/4G modem online for a couple of days should something happen to the main battery bank (discharge, fuse blown, flooding...). I've looked at "battery isolators", which are just a pair of diodes, but I'm unhappy about the 0.7V voltage drop these introduce, and "automatic charging relays", which don't have the voltage drop but won't keep the backup battery connected (and charged) when not charging the main bank (meaning the computer would run off the backup battery when not charging). I'm also concerned that such a set-up would overcharge the backup battery when charging the main bank. It seems to me what I need is a buck/boost converter and separate charger, which can take the (nominal) 12V from the main bank and provide a ~13.5V float charge for the backup battery, while preventing the backup battery from reverse powering the house system should this drop out. Anyone know of such a beast?

[BradC]

Check out some caravanning sites and you'll find more than one to choose from.

[Lomax]

Check out some caravanning sites and you'll find more than one to choose from.

Sorry, spent hours looking and no dice. All I can find are the two items mentioned above (isolating diodes and relays), neither of which will work in my case. I have also found a number of 12V > 13.8V DC/DC converters, which is an interesting option, but without current limiting I reckon there's a fair chance the little SLA would die if connected at a discharged state. Besides, I would need a DC/DC converter that can provide a reliable 13.8V output at input voltages between (say) 11.5V (discharged main battery bank) and 14.4V (main bank equalisation charge). All the ones I've seen specify 12V input only.

[Lomax]

To clarify, I imagine the following scenarios.

Main battery bank at nominal voltage (e.g. 11.5-12.8V):
Backup battery is float charged at 13.8V

Main battery bank is being charged at 13.8V:
Backup battery is float charged at 13.8V

Main battery bank is being charged at 14.4V:
Backup battery is float charged at 13.8V

Main battery bank is in a discharged state:
Backup battery supplies power to security system until discharged

Main battery bank is disconnected:
Backup battery supplies power to security system until discharged

Backup battery is fully discharged when power is restored:
Charge current going into backup battery is limited to a safe level

[mjrandle]

Some ideas to consider:

https://www.redarc.com.au/battery-chargers/in-vehicle-chargers

 or

http://www.tekonsha.com/products/brake-controllers/breakaway-kits/charger/TzaIxkVx1cL5ERqvVYjzzC8%7CG2H%7Cr2aKfMWTeCI5U0E=

The beauty of Redarc unit is that it is only enabled when it receives an input on ignition terminal and draws minimal current when in standby.


Cheers,

Mike

[Lomax]

Some ideas to consider:

https://www.redarc.com.au/battery-chargers/in-vehicle-chargers

 or

http://www.tekonsha.com/products/brake-controllers/breakaway-kits/charger/TzaIxkVx1cL5ERqvVYjzzC8%7CG2H%7Cr2aKfMWTeCI5U0E=

The beauty of Redarc unit is that it is only enabled when it receives an input on ignition terminal and draws minimal current when in standby.


Cheers,

Mike

Ah. Aaaah. Aaaaaaahhh. Never seen this kind of kit here in Europe - I think we use a simple steel cable braking system on break away trailers, though I don't drive so not sure. In any case, the Tekonsha 2024-07 Battery Charger looks like the ideal piece of kit; wide input voltage range, automatic float charge, temperature compensated, and with 4A output current. Sweet! Thanks for the great tip! 

Edit: Re. the Redarc unit - I want the backup battery to receive a charge as long as the main battery bank is at a safe discharge level, not just when the engine is running. The main bank will also be charged from solar panels, a small wind turbine (300W) and shore-power when available.

Edit 2: I see now the Tekonsha charger only has an 800mA output, that's probably not going to be quite enough My 3G/4G router draws 75mA at idle (connected) and probably 200mA when sending/receiving, add a Raspberry Pi Zero w. 5V DC/DC converter and a few sensors and there's not going to be much current left to charge the battery... Hmmm...

Edit 3: The Redarc BCDC-1206 unit looks really nice, compact and powerful; at 6A output it can definitely supply enough juice. But blimey, £200 including postage - plus inevitable customs charges. Grrrrr.

[oldway]

What you want to do is a DC no break feeding.
You must feed the critical load by the 2 batteries (main and back up) through 2 diodes.
To reduce voltage drop, you can choose schottky diodes.
To charge the back up battery, you need an additive converter.
It is an isolated dc/dc converter with 11 to 15V input voltage and output from 0 to 3V.

[Lomax]

Having spent most of last night searching for a solution without finding anything that particularly tickled my fancy, I suddenly stumbled upon the OpenUPS from Mini-Box. This appears to be the perfect solution, with lots of bells & whistles. I particularly like that the Vin threshold for switching over to backup battery power is configurable, and that charge state and Vin can be polled via USB (and I2C, and SMBUS!). And when I managed to find a second-hand unit on the bay for just £50 delivered I jumped in. It may not be the most power efficient option, but then multiple DC/DC conversions are never going to be free. I will set the output voltage to 12V for powering the router, and stick a 5V DC/DC converter on that for powering the Pi. It all goes into a small, innocent looking, watertight ABS wall box which will be mounted out of view, with its own internal (backup) 3G/4G antenna. To stop the boat from transmitting alarms, position and other telemetry you'd have to find this box, understand what it is, break into it and disable the system. Anyone stealing my boat will wonder how the hell the police managed to find them so quickly

[oldway]

According to the general principles, the more components, the less reliable.

At the level of reliability, it is difficult to do better than the very simple solution that I proposed that uses only a few diodes.

[mjrandle]

Hi Lomax, thanks for the link to OpenUPS - it looks very interesting.  Note that both the Redarc BCDC1206 and the OpenUPS use the same LTC3780 buck-boost controller.  The unfortunate thing about the BCDC1206 is the price and that you need be very careful not to forget to disconnect both batteries before tightening terminals - it's very easy to short out the terminals to the grounded case with your screwdriver.

You could also hack something together from the cheap LTC3780 modules that you can buy on Ebay.  The low voltage cutout should be easy to implement - Silicon Chip magazine have a battery lifesaver project which may be of interest to you.  For an off the shelf solution Redarc (I have no association, just a customer) VS12 voltage sensitive relay.


Cheers,

Mike

[Codebird]

I've looked at "battery isolators", which are just a pair of diodes, but I'm unhappy about the 0.7V voltage drop these introduce.

Look into 'ideal diodes' - they are readily available as assembled modules. They use some active electronics to emulate the performance of an ideal, zero-drop diode - essentially a comparitor and a MOSFET. They are intended for exactly this sort of application.

[oldway]

You can use schottky diodes (only 0.4V voltage drop)
14.4V equalizing recharge voltage may be too high, using a diode in serie with battery may be usefull to protect the load against overvoltage.