High performance DIY Battery maintainer.

[merl]

I posted details of my DIY battery maintainer on here a while ago,

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...

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

 

[barryd]

I can't say I understood all that but it sounds great. Sign me up! Well you already have

Will it have any conflict with the Sargent system which on our van when on EHU charges both the leisure and cab batteries and seems to decide itself which one to swap to when on hookup?

 

[merl]

In a word....No

 

[winks]

Does exactly the same job as my Victron Cyrix that I got with an Amazon voucher I won on the forum a few years ago.

Never had a dead starter over winter lay up since it went on. I like your gizmo mind.

Cheers

H

 

[trevskoda]

My votronic keeps all batts up, and i have a switch running a 200amp re;ay to connect starter to les batts when driving, this can also be used for a jump start if i have forgoten to turn h lights out and run the starter batt down, also can jump start other folks van/car if required, and have done so in the past.

 

[merl]

My votronic keeps all batts up, and i have a switch running a 200amp re;ay to connect starter to les batts when driving, this can also be used for a jump start if i have forgoten to turn h lights out and run the starter batt down, also can jump start other folks van/car if required, and have done so in the past.

There's always various ways to tackle the same subject Trev and I do like what you've done.

 

[wildebus]

Your missing the point AND the functionality on the AMT12-2 for a start.
You stated something along the lines of "voltage drop of 0.2V"? and the battery master is 0.6V, whereas your device is 0.1V.
The objective of those battery maintainers is not to maintain the starter and the leisure battery voltages to be as close as possible to each other, so using that criteria as a comparsion to how well it works is flawed greatly.

As a device to equalise a starter batteries voltage to be as close as possible to a leisure batteries voltage, it may be great, but don't confuse it to a multi-function battery maintainer such as an AMT12-2 which has, for example, built-in protection to prevent over-discharge of the leisure battery while maintaining the starter battery.
P.S. And if you wanted to have the AMT in the mode of voltage equalization (which is a SELECTABLE option, not the only mode), the voltage drop I have measured is under 0.1V and uni-directional as well.

if you want a "clive mott" solutiion that is fit and forget, just swap the fuse for a PTC. If you want a proper solution, get a proper one.

 

[Pudsey Bear]

I quite like it for my needs, it's cheap and essentially effective, but what I know.

I did for the Clive Mott to most of my vans, but I like the modernity of the Gizmott.

 

[bartman]

I fitted one of theTec3m relays years ago, dead easy to fit and it's always done the job. No half-arsed pulling fuses out each time you start a la Clive Mott either!

 

[wildebus]

If someone wanted a "fix and forget clive mott" it would be incredibly easy anyway. I'm a bit surprised he never updated his solution to do this (or did it to start with).

How:
Wire up "mott" as indicated, but in line with the cable use a 5-pin relay - available anywhere on-line for around £5 each delivered or in bundles for around a quid each.
Connect one side to pin 30, the other to pin 87a.
You then connect pin 85 to a ground and pin 86 to an ignition signal (one usually found by the radio for example)

Then what happens is the two batteries are connected, but as soon as the ignition is turned on, the relay activates and the connection is disconnected. Then when the ignition is off again, the connection is remade.

Of course, when parked up and USING the van, you could be potentially drawing from the starter battery as well as the leisure battery so not a great solution either, but at least you don't go through a box of fuses every year

 

[Pudsey Bear]

I could see no fuses other than next to the batteries, but I'd fit them to anything connected directly to protect the cables.

 

[trevskoda]

The reason i re wired my van with a relay and dash switch is because from factory it was fitted with a diode unit to charge the back step unit.
Problem was it only passed 13.7v which is cr-p, also if i started the engine with all batts connected it will slip and break the fan belt, happened once.
So i start up normal and then flick the switch once a bit of reves are on the donkey, what i do have to rememeber is to switch of when i have stopped so i wont run batts down for starting or bust the belt as its a pain in ones dark zone to refit a replacement.
I could have riged it to altanator triger wire but again i prefair some revs on the engine rather than chance the belt, i have thought of a timmer device, may or may not happen and also leave the switch for jump starting others.

 

[merl]

You stated something along the lines of "voltage drop of 0.2V"? and the battery master is 0.6V, whereas your device is 0.1V.
The objective of those battery maintainers is not to maintain the starter and the leisure battery voltages to be as close as possible to each other, so using that criteria as a comparsion to how well it works is flawed greatly.

Nope, Voltage drop is massively important if you want your starter battery to last as long as possible, there's a wealth of knowledge surrounding the importance of charge/float/maintenance voltages of lead acid batteries, why on earth would it be preferable to have your starter battery sitting at a float/maintenance voltage lower than your LA leisure battery?

As a device to equalise a starter batteries voltage to be as close as possible to a leisure batteries voltage, it may be great,

It is, and it was designed that way. Thanks

but don't confuse it to a multi-function battery maintainer such as an AMT12-2

I'm not confusing anything, just simply comparing the voltage drop figures which are better than many others with the Gizmott. (See above about the importance of lead acid battery float/maintenance voltage or look it up)

I have measured is under 0.1V and uni-directional as well.

Pretty similar to the Gizmott then, I wonder why they went to the trouble of getting it so low if it's not actually important?

Your missing the point

Nope, YOU'RE missing the point David. I've posted a really inexpensive simple little DIY battery maintainer who's performance WRT voltage drop (See above for the importance of voltage drop or look it up) for those who fancy building building such a device at low cost. Nothing more. You appreciate the versatility of the Ablemail, that's fine the Ablemail is brilliant but from a DIY perspective versatility isn't particularly important because a DIYer simply builds the unit as needed for their current system.

if you want a "clive mott" solutiion that is fit and forget, just swap the fuse for a PTC.

Nope, doing that will only solve the 'over current during starting' issue and still doesn't cover accidental discharge of the starter battery so you'll have to pull the fuse....hardly fit and forget. To stop back discharge you'll need a diode of some description and suffer the inevitable downside introduced by it's voltage drop.

If you want a proper solution, get a proper one.

I have got one thanks, I made one and it's as good as any re voltage drop and better than many.


Anyway David, how comes it took you nearly 13 hours to chip in with your predictable disparaging comments re one of my electronics posts? You're getting slow in your old age, you need to up your game mate

 

[Fisherman]

Some folk need to stop talking Urdu on here.

 

[wildebus]

Anyway David, how comes it took you nearly 13 hours to chip in with your predictable disparaging comments re one of my electronics posts? You're getting slow in your old age, you need to up your game mate

I've other things to do then disparge your jokey devices as soon as you post them. (wielding a paintbrush to do my decking ).
If I wanted a POS solution, I would stick with the Motty option instead of an overpriced aliexpress based one, but no thanks to both

 

[merl]

I've other things to do then disparge your jokey devices as soon as you post them. (wielding a paintbrush to do my decking ).
If I wanted a POS solution, I would stick with the Motty option instead of an overpriced aliexpress based one, but no thanks to both

Your electronics knowledge really shone through on that post David. I particularly liked the way you backed up your initial criticism and explained why voltage drop wasn't particularly important.(No... don't tell me.... you don't have time and you've got better things to do?)
That's a much better effort too, less than half an hour. Well done and keep up the excellent work
Yawn Yawn.

 

[Pudsey Bear]

It's all work of the Divil boys.

 

[B4cus]

The design is very simple, here's the schematic
View attachment 141344
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.
View attachment 141371

Great idea and one I'm about to copy.
I'm just a bit confused by the build photo as one end of the bulb assembly appears to go to the leisure battery, whereas the circuit shows it connected to the starter battery... Or am I seeing things?!

I'm assuming the bulbs are acting as current limiters. In which case it probably doesn't matter whether they are on the input or output

 

[merl]

Great idea and one I'm about to copy.
I'm just a bit confused by the build photo as one end of the bulb assembly appears to go to the leisure battery, whereas the circuit shows it connected to the starter battery... Or am I seeing things?!

I'm assuming the bulbs are acting as current limiters. In which case it probably doesn't matter whether they are on the input or output

Hiya, yes absolutely it actually doesn't matter which comes first, the diode or the bulbs but didn't see the point in mentioning it in the description for fear of making the post even more long winded. Well spotted by the way
Both fuses should be at the source in case you hadn't noticed that slip up too BTW.
Merl