merl
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I posted details of my DIY battery maintainer on here a while ago,
wildcamping.co.uk
I've had it installed in my van for a few years and although it's worked well, keeping my starter battery topped up over a couple of winters the improvement of semiconductors and the drop in prices lead me to revisit the design and produce something better and cheaper, this new version costs less than £10 to make from easily available items and you only need basic soldering skills to be able to make it.
Just like any commercial battery maintainer or Clive Mott fuse it fits between the leisure battery and the starter battery effectively connecting them together so that both batteries can be kept maintained by either solar or EHU.
It's performance is excellent, it keeps the starter battery maintained with less than 0.1V difference between the two batteries, voltage drop is important because Lead Acid starter batteries degrade if their voltage is allowed to drop significantly for long periods of time such as during lay up over the winter, if you have lithium leisure batteries the voltage drop of the maintainer isn't quite so critical and there's an even cheaper and simpler alternative that's mentioned toward the bottom of this post, here's how some commercial units compare to this little Gizmo.
Votronic standby charger 0.6V
Battery Master 0.6V
Sterling battery maintainer 0.3V
Ablemail AMT2 0.2V
Clive Mott fuse 0V
Nothing beats the fuse WRT voltage drop (or cost) but it has a massive drawback in that you must fit the fuse when you park up and remove the fuse again before you start the engine, if you don't you could either blow the fuse or worse discharge your starter battery while stationary leaving you unable to start the van. Just like the commercial devices, my Gizmo is 'fit and forget', it passes power from the leisure batteries into the starter battery to make up for losses due to self discharge, alarm drain, radio drain etc and keeps it maintained from the main leisure battery which should of course be itself kept topped up with solar or EHU. It also doesn't allow the starter battery to be drained via the leisure battery.
The design is very simple, here's the schematic
It consists of a 15A ideal diode module and a self heating PTC thermistor which is made from 3 or 4 50W 12V capsule lamps in parallel.
Here's a link to the diode module on Ebay https://www.ebay.co.uk/itm/195052373280 They are slightly cheaper on Ali Express
Here's the 12V halogen capsule lamps on ebay https://www.ebay.co.uk/itm/266846258153 Amazon have a pack of 10 for £5
Here's the plastic wiring box to fit everything in. https://www.screwfix.com/p/debox-24a-in-line-junction-box-50-x-102-x-28-5mm-white/8692h
BUILDING
The module looks like this, the cotton bud is for scale.
The leisure battery + is connected to the 'IN' pad. Either of the pads marked - is taken to battery negative.
The lamps need to be connected in parallel, I bent the pins over and soldered them together with two short lengths of stout copper wire, a blue tak support helps. This gives two connections, one connection goes to the 'OUT' pad of the module and the other lamp connection becomes the starter battery output.
Below is the finished unit in a Dbox.
Points to note.
The unit is a maintainer not a charger. It'll supply whatever is required by the starter battery up to around 3A to keep it topped up but it should not be used to charge a flat battery.
A 5A fuse should be fitted close to the unit on the leisure battery input and another 5A fuse close to the starter battery on the starter battery input.
The unit works perfectly with lead acid leisure batteries. I have lithium leisure batteries and used the Gizmo in my van over the last winter layup however I personally lower my lithium charge settings to 13.5 volts to maintain/store the lithiums at around 70%, again it works great at or around this kind of voltage. If, like most people with lithium you don't mess with the storage voltage over winter and leave your settings at or above 14V then the minimal voltage drop of 0.1V could be slightly too good and you could replace the Ideal Diode Module with three or four 1N4820 Schottky diodes wired in parallel, this will maintain the starter battery at around 0.25V lower than the lithium leisure battery so approximately 13.35V and will save a couple of quid too.
Happy soldering
Charging cab / engine battery from Solar panel.
Ive often left all my vehicles for months on end with no charge. Cars get left for up to five or six months, bikes often the same and the motorhome often gets left over winter. I just disconnect the negative lead off the batteries. Seems to work. Ask any good battery shop and you will be told...
Just like any commercial battery maintainer or Clive Mott fuse it fits between the leisure battery and the starter battery effectively connecting them together so that both batteries can be kept maintained by either solar or EHU.
It's performance is excellent, it keeps the starter battery maintained with less than 0.1V difference between the two batteries, voltage drop is important because Lead Acid starter batteries degrade if their voltage is allowed to drop significantly for long periods of time such as during lay up over the winter, if you have lithium leisure batteries the voltage drop of the maintainer isn't quite so critical and there's an even cheaper and simpler alternative that's mentioned toward the bottom of this post, here's how some commercial units compare to this little Gizmo.
Votronic standby charger 0.6V
Battery Master 0.6V
Sterling battery maintainer 0.3V
Ablemail AMT2 0.2V
Clive Mott fuse 0V
Nothing beats the fuse WRT voltage drop (or cost) but it has a massive drawback in that you must fit the fuse when you park up and remove the fuse again before you start the engine, if you don't you could either blow the fuse or worse discharge your starter battery while stationary leaving you unable to start the van. Just like the commercial devices, my Gizmo is 'fit and forget', it passes power from the leisure batteries into the starter battery to make up for losses due to self discharge, alarm drain, radio drain etc and keeps it maintained from the main leisure battery which should of course be itself kept topped up with solar or EHU. It also doesn't allow the starter battery to be drained via the leisure battery.
The design is very simple, here's the schematic
It consists of a 15A ideal diode module and a self heating PTC thermistor which is made from 3 or 4 50W 12V capsule lamps in parallel.
Here's a link to the diode module on Ebay https://www.ebay.co.uk/itm/195052373280 They are slightly cheaper on Ali Express
Here's the 12V halogen capsule lamps on ebay https://www.ebay.co.uk/itm/266846258153 Amazon have a pack of 10 for £5
Here's the plastic wiring box to fit everything in. https://www.screwfix.com/p/debox-24a-in-line-junction-box-50-x-102-x-28-5mm-white/8692h
BUILDING
The module looks like this, the cotton bud is for scale.
The leisure battery + is connected to the 'IN' pad. Either of the pads marked - is taken to battery negative.
The lamps need to be connected in parallel, I bent the pins over and soldered them together with two short lengths of stout copper wire, a blue tak support helps. This gives two connections, one connection goes to the 'OUT' pad of the module and the other lamp connection becomes the starter battery output.
Below is the finished unit in a Dbox.
Points to note.
The unit is a maintainer not a charger. It'll supply whatever is required by the starter battery up to around 3A to keep it topped up but it should not be used to charge a flat battery.
A 5A fuse should be fitted close to the unit on the leisure battery input and another 5A fuse close to the starter battery on the starter battery input.
The unit works perfectly with lead acid leisure batteries. I have lithium leisure batteries and used the Gizmo in my van over the last winter layup however I personally lower my lithium charge settings to 13.5 volts to maintain/store the lithiums at around 70%, again it works great at or around this kind of voltage. If, like most people with lithium you don't mess with the storage voltage over winter and leave your settings at or above 14V then the minimal voltage drop of 0.1V could be slightly too good and you could replace the Ideal Diode Module with three or four 1N4820 Schottky diodes wired in parallel, this will maintain the starter battery at around 0.25V lower than the lithium leisure battery so approximately 13.35V and will save a couple of quid too.
Happy soldering
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