Leisure battery ammeter question

[bongojoe]

I've just fitted a dual voltmeter / ammeter to the leisure battery in my Bongo (one of these to be precise). It works great, shows the charge in numbers and gives me some sense of how much power I'm using to keep my beer cold and when the child incessantly runs the blinds up and down and up and down and up...

But... I've just noticed that when I turn the ignition key to the 'ON' position (ie. before starting the engine) the amps shoot up to 15A or so, then drops down as the orange coil light goes off and stays at about 3-4A.

How is this possible? There is a split charge relay between the two batteries and if I turn on the headlamps (which still run off the main battery) no power is drawn from the leisure side.

The ammeter measures on the negative side of the battery with a 75mV shunt. As this is common to both batteries, could that be something to do with it?

[g8dhe]

It will indeed be showing rubbish if you have a common earth in which your measuring the current from both batteries! One could be charging and the the other discharging and the results will tend to cancel out! You need to fit the shunt in the earth running to the specific battery you want to measure.

[bongojoe]

It will indeed be showing rubbish if you have a common earth in which your measuring the current from both batteries! One could be charging and the the other discharging and the results will tend to cancel out! You need to fit the shunt in the earth running to the specific battery you want to measure.

Sorry, perhaps I didn't explain that properly. The shunt is fitted between the negative/earth terminal on the leisure battery and the lead that was connected to that terminal, and the ammeter measures across that connection. What I meant by 'common to both batteries' is that, unlike the positive side which is separated by the split relay the negative side of both batteries are always connected to each other through the body of the vehicle.

Does that make sense?

The meter seems to accurately measure power drawn by the fridge, stereo, blinds, etc. The only anomaly is this surge of power when the ignition is in the 'ON' position.

As usual, I seem to know enough about this stuff to get myself into trouble but not enough to get out!

[g8dhe]

Right, yes the word "common" has a specific meaning in electrical/electronic parlance!

If your getting a reading as soon as you turn the Ignition key to the normal running position, then that implies that you have a relay switched from the Ignition rather than a Voltage Sensitive Relay setup, this is quite a normal method, I use it myself in preference to a VSR where you have no control as to when the relay operates and will be the current from the S/B flowing into the L/B charging it because the L/B has had some use. while the Ignition was off.
Depending on how the meter reads the current is quite normal, just confirm the following, with the Ignition OFF does the current meter show a +reading or a -ve reading when you use power to say the fridge or anything else connected to the the L/B ?

[Velocette]

It will indeed be showing rubbish if you have a common earth in which your measuring the current from both batteries! One could be charging and the the other discharging and the results will tend to cancel out! You need to fit the shunt in the earth running to the specific battery you want to measure.

Sorry, perhaps I didn't explain that properly. The shunt is fitted between the negative/earth terminal on the leisure battery and the lead that was connected to that terminal, and the ammeter measures across that connection. What I meant by 'common to both batteries' is that, unlike the positive side which is separated by the split relay the negative side of both batteries are always connected to each other through the body of the vehicle.

Does that make sense?

The meter seems to accurately measure power drawn by the fridge, stereo, blinds, etc. The only anomaly is this surge of power when the ignition is in the 'ON' position.

As usual, I seem to know enough about this stuff to get myself into trouble but not enough to get out!

It sounds like the meter is doing what it is designed to do. Presumably there is a reason why it has to go in the Negative side? It will measure current drawn from either battery as is as said above, I think you have to ignore the effects that you know are the starter battery. Tricky thing is if you are worried by a small but unknown discharge, the meter is no use to you. For keeping an eye on your "known knowns" like your fridge and radio etc it is fine.

Incidentally, I have just a switchable analogue voltmeter that I fitted and as per the instructions I installed a negative bus so the chassis isn't the only -ve path. Probably unecessary as the thing I fitted was meant for a boat!

[Rebiam]

looks like a neat piece of kit - I am not so electrically minded. Is this the best sort of gauge for keeping an eye on your leisure battery condition / usage?

[bongojoe]

Right, yes the word "common" has a specific meaning in electrical/electronic parlance!
If your getting a reading as soon as you turn the Ignition key to the normal running position, then that implies that you have a relay switched from the Ignition rather than a Voltage Sensitive Relay setup, this is quite a normal method, I use it myself in preference to a VSR where you have no control as to when the relay operates and will be the current from the S/B flowing into the L/B charging it because the L/B has had some use. while the Ignition was off.

That sort of makes sense, I think...! Here's a photo of the innards of the black box which sits between the two batteries - any further insights on exactly what it's doing and when would be much appreciated:

Depending on how the meter reads the current is quite normal, just confirm the following, with the Ignition OFF does the current meter show a +reading or a -ve reading when you use power to say the fridge or anything else connected to the the L/B ?

The meter doesn't seem capable of showing a negative reading. When the engine is running or my new solar panel (I'll come on to that later!) is charging the L/B the meter reads '0'. When it's dark, the ignition is OFF and the fridge is running, the meter shows a current of 2-3A. Using the blinds, cranking up the stereo, etc., all make that reading increase as you'd expect.

looks like a neat piece of kit - I am not so electrically minded. Is this the best sort of gauge for keeping an eye on your leisure battery condition / usage?

Don't know, but it looks clever and there's a perfect spot for it just under the steering wheel!

[g8dhe]

Difficult to tell just with a photo of relays! But no electronics so probably not a VSR, two relays so does it also have a Low voltage Cut-out perhaps ?

If it can't show a -ve reading then you won't know if your ever charging the L/B! and in that case its a bit suspect why the L/B is showing a heavy discharge at the start unless the S/B is flat - Ah Ha you have a Solar panel that would charge the L/B and not the S/B? Given that the meter shows 15 Amps that sounds like its showing the almost the full Glow Plug current, in which case the S/B is not supplying anything until you turn to the Starter position - something seems odd here, unless a lot more wiring changes have been made to fit the solar panel as well ?

[bongojoe]

The solar charger is connected directly to the terminals on the L/B. After I installed it, I left the fridge running for over a week without using the van and the battery stayed fully charged, so it's clearly doing what it's supposed to.

The solar charger has its own display which shows L/B voltage and indicates when the panel is sending power to the battery, but agree an ammeter which also showed a negative reading would be helpful. Perhaps I need an upgrade...! Or perhaps I could devise a clever switch which reverses the polarity across the shunt...

Re. the relays, I may open the box up again, see where the wires go and try and figure out what's going on - visually I should be able to see when they're actuated. Could also remove the inline fuse on the wire which charges the L/B from the S/B and see if I still see a current when ignition is ON.

Thanks for your expert help, g8dhe. It's all good fun - will report back!

[bongojoe]

Had another poke around...

Both relays appear to simply be activated when the ignition is in the 'ON' position, thus connecting the +ve side of the two batteries. So that would explain why power is being drawn from the L/B when the glow plugs are heating up. If I remove the fuse on the cable between the two batteries no power is drawn when the ignition is on.

Any point adding a diode to only let power flow to the L/B, or is it a non-issue?

I also tried reversing the polarity of the ammeter across the shunt and it does indeed register the charge going into the battery when the engine's running. So I'll add a switch next to the meter to allow me to reverse the polarity - flick it one way to see how much juice the solar panel is sending to the L/B during the day, and the other way to see how much I'm using to keep my beer cool during after dark.

[g8dhe]

No its a non issue! I personally prefer that type of operation, and as you have a solar panel its even better for you - say for instance you do end up with a flat S/B, but a charged L/B, then simply turn on the ignition and leave for ten minutes or more, the power in the L/B will charge the S/B, now turn to Start position, the relay drops out so only the S/B is trying to turn the starter motor over, when you release the Ignition key the relay will operate again and connect the two batteries allowing both to be charged by the alternator. If you had the VSR type then your stuck, you can't control the relay, you will need a jumper cable to link the two batteries.

One word of warning if the two relays are in parallel which is what you suggest, then whilst the total current carrying capacity of the relays is doubled, the break and make currents are not affected and the rating is the same as a single relay, now normally its the break/make which cause arcing and damage to the contacts which is why split charge relays are normally rated at 40Amps and higher, 100 Amps is not uncommon, so be aware if you get any failures in this area its better to have a single higher rated relay rather than multiple relays on parallel.

[bongojoe]

Thanks, Geoff - very informative and all makes perfect sense.

[Jaws]

looks like a neat piece of kit - I am not so electrically minded. Is this the best sort of gauge for keeping an eye on your leisure battery condition / usage?

No, not really. As has been pointed out this unit only indicates charge and not discharge unless an additional switch is put in. It doesn't include the shunt either. But it is very cheap and useful to check that the LB is actually charging when expected (or unexpected!).
I use a NASA BM1 which indicates charging or discharging with a permanent (LCD) display of state of charge. It will also switch to provide an indication of time to discharge or full charge respectively according to load or input. So it will tell you to within an hour or so when it is time to drink your beer before it gets warm It is designed for boats/nautical use so is priced accordingly. But it did include the shunt, ample wiring and good instructions on how to use it, besides being bullet proof and safe to 10m under water
It was this device that told me long before the lights on the dash came on that the alternator was failing. I'm sure there are similar kits around for less money but you get what you pay for.

[bongojoe]

Jaws - the NASA BM1 looks the business! Added to my Christmas list.

Think I'll buy a better/newer leisure battery first though, which should be a bit cheaper. The one I have is still the original Mazda battery, so only 75Ah and probably not even a proper L/B.

The polarity-reversing switch is fitted and working well. Very reassuring to see juice going back into the battery when driving along or soaking up the sun!

[tallbongo]

Any point adding a diode to only let power flow to the L/B, or is it a non-issue?

It doesn't sound like you intend to, but don't add a diode. As well as the points g8dhe makes, the voltage drop across it will impact the charging effectiveness.

It's pretty cheap and easy to replace the LB charging circuit. I recently replaced the unreliable rat's nest that was installed in my Bongo (and certain to fail the new MOT regs) with a 75A relay, 200A rated cable (though I'm dubious that it really is) and 60A strip link fuses. Total cost for the parts was ~£20. Although yours looks more professional than the mess I had, is the wiring thick enough for the potential currents? My old setup had a 30A fuse (in a 10 or 20A rated holder ) that blew on several occasions when the LB was flat. Your solar charger may have prevented you running into this problem so far.