Leisure battery ammeter question

[bongojoe]

I don't think the wiring between the S/B and L/B on my conversion is up to much. Not sure how I'd figure out the rating of the cable but I wouldn't describe it as fat. There is a 30A inline fuse, in a standard-looking white plastic holder.

I probably will replace it all at some point.

[lazyb5]

Any chance of a write up in English for us non techie folks, would love to have a couple of gauges telling me the volt and amp of both my batteries, but couldn't really follow the thread what with shunts and things

[Jaws]

Any chance of a write up in English for us non techie folks, would love to have a couple of gauges telling me the volt and amp of both my batteries, but couldn't really follow the thread what with shunts and things

OK here goes; the gauges described here display voltage by measuring the potential difference between the positive terminal of the battery and the negative terminal. In practice this usually means one wire to the positive terminal (or connected to another wire taking power from the battery to be monitored) and another to the negative terminal or the chassis (which is connected to the negative terminal via a thick wire.) That's volts.
A shunt comes in when you want to measure amps. Basically it is a resistor, usually placed between the battery negative terminal and earth (the chassis). Generally it consists of a metal strip carefully adjusted to a precise resistance eg 75 Ohms. Remembering your physics you will recall V=I*R or rearranged I=V/R. So the gauge measures the voltage drop across the resistor (shunt) and since the resistance is known/fixed uses this voltage to calculate the current being drawn. So the gauge needs around three or four wires to be connected and a resistor (shunt) between the negative terminal and the chassis to give the information required. The BM1 can also be programmed with the battery capacity eg 100AH and over time will be able to predict (roughly) how long to full discharge when current is being drawn at a given rate. Useful if camping without hook up.
I'm not sure if there are units designed to monitor both batteries but most folks are interested in the LB and so this is usually where a battery monitor is connected. Hope that helps.

[bongojoe]

I drew you a picture!

[lazyb5]

Thanks chaps really useful, i`ve just suffered a flat starter battery due to my leisure being flat and low on fluid so the engine was trying to charge the leisure more than the starter. Combine that with short trips and a light left on. I would like to be able to see volts and charging amps on both batteries rather than what i was drawing from the leisure. I rarely use the van for real camping. Again many thanks.

[Jaws]

Hi Lazyb5; no problem but it sounds like you have an issue that won't be cured with a battery monitor! A good split charge system should not allow the situation you describe to develop; that's why many use a Voltage Sensing Relay (VSR). This will only connect the LB to the SB once the voltage of the SB gets to around 13.5V+ i.e. fully charged. So even short trips should be enough to keep the SB topped up. You may have a system that just connects the batteries together when the ignition is on?
Again, a well functioning alternator can generate up to 90Amps and regulate to up to 14V; more than enough to boost both batteries over a 5-10mile trip, although longer to fully charge both. Are you sure your sure your alternator is doing it's job
BTW the BM1 (or similar) monitors charge and discharge so when the LB is being charged from the alternator you effectively see the voltage and current being delivered by the alternator via the split charge system. If this is good it is unlikely the SB is flat unless the alternator is failing...

[tallbongo]

This will only connect the LB to the SB once the voltage of the SB gets to around 13.5V+ i.e. fully charged.

Once the engine is running the alternator will provide ~14V to the starter battery thus opening a VSR regardless of the charge state of either battery.

The VSR just uses a different method to connect the two batteries, i.e. sensing the voltage to tell it that the alternator is on, as opposed to a control signal from the alternator or switched live.

[Jaws]

Thanks for the correction I guess the point is a VSR or alternator switched relay will only connect the LB once the alternator is running. So a discharged LB is isolated until the SB has finished running up the glow plugs and cranking the starter. Then there should be plenty of power to charge both even on a short run.

[g8dhe]

Not quite even that, the relay on Ignition controlled setup isolates the L/B during the actual Starter phase, so its only connected when in running mode whether or not the engine is running. The nice thing about this is that you can let the L/B charge the S/B if you should have flattened that for some reason.

[Jaws]

Not quite even that, the relay on Ignition controlled setup isolates the L/B during the actual Starter phase, so its only connected when in running mode whether or not the engine is running. The nice thing about this is that you can let the L/B charge the S/B if you should have flattened that for some reason.

Sorry Geof I don't quite understand; do you mean that when the starter motor is being cranked i.e. ignition switch turned to 'start' position then the LB is isolated? I would expect that since the relay circuit is unlikely to be able to take the current draw of the starter motor. But when the ignition switch is just turned to the 'on' position (for glow plugs etc) it is connected to the SB? I think this was my point. A discharged LB at this stage would drain the SB and possibly lead to a failure to crank the starter

[bongojoe]

Jaws - that's exactly how mine seems to be set up.

Current flows from the L/B when ignition is at 'ON' but I guess if I was worried about starting with a flat L/B I guess I could take the fuse out before ignition to be in te safe side.

[g8dhe]

Yes that's correct, there are two Ignition circuits involved the relay should be connected using the IG2 circuit NOT the IG1 circuit, that;s the Blue/White wire;

[lazyb5]

Hi guys your starting to loose me again, I have the willington split charge kit. The voltage on the starter battery terminals is 12.45 and 12.3 on the lb. this is without ignition. After starting engine at idle, I have 14.2 at starter and 14.2 at leisure. If I fitted the gauges as described, I'd be able to see if I have anything that is discharging either battery.

[Jaws]

Thanks chaps really useful, i`ve just suffered a flat starter battery due to my leisure being flat and low on fluid so the engine was trying to charge the leisure more than the starter. Combine that with short trips and a light left on. I would like to be able to see volts and charging amps on both batteries rather than what i was drawing from the leisure. I rarely use the van for real camping. Again many thanks.

Willinton= good
If you fit a Battery Monitor (BM) to the LB, as soon as the SB and LB are connected you will be able to see what is happening to the pair. The Voltages you have quoted seem appropriate for your set up. If you have experienced a NO GO situation I would get good batteries before a good battery monitor. And check the alternator.

[bongojoe]

Update on this (and more questions)...

I decided to rewire the split charger earlier this week. Took out several lengths of domestic two-core flex, the type you might plug a portable telly into the wall with, which was connecting the two batteries together. Replaced all that with 40A automotive cable and the weird dual-relay pictured earlier in this thread with one of these 40A versions from Maplin. Also installed a 30A fuse at each battery end.

Tested everything as I went along and all working well. However yesterday I noticed (care of my trusty Chinese ammeter) that the alternator wasn't charging the L/B when the engine was running. Took a look and found one of the 30A fuses blown.

Replaced it and all working OK again, but then noticed that the draw on the L/B when I turn the ignition to 'ON' shoots up to well over 20A before dropping down to 12-14A, a massive leap on what I reported when I started this thread and getting very close to the fuse rating which probably explains why it blew.

I've also replaced the L/B with a new 110Ah version since starting this thread.

So my question... why would the load have increased so much? Is it because my L/B is now much larger than my S/B, or is it because I've installed a more chunky cable? Or is something else going on?