for the electricians amongst you

[horsefeathers]

In my van i have a secondary 12v battery that i wire up to an invertor to give 240v in order to run my key cutitng machine. Works fine! (a 240v machine is more versatile than a 12v one in my opinion)

Usually i charge battery fairly regularly depending on how many keys I think I have cut since last charge. However, to remove guess work, i would like to incorprate something in the circuit (before the invertor) that could give me an indication of how much power the battery has left before it needs charging. Once or twice I have been merrily cutting away only for the invertor to squeal and the motor stops. The battery now needs charging before it will run the cutter again (obviously).

Should I be looking for a voltmeter that can be placed 'in line' so to speak that always reads the battery voltage when the battery switch is enabled (would it use any power itself??), , and would it be worth it if 12v runs the invertor but say 11.5v fails to? The guage would hardly move between the two figures. Or should I be thinking in terms of amps and get an ammeter in line (as if I know what an amp is!!). Maybe amps would give a more accurate indication of battery level so that I could notice a greater swing between fully charged and close to failure!
--------------
________________ ___ ______ ???? ________| |____
| _________ | | ______ ???? ________| invertor |___
| | | | ---------------
__________ -----
| | Batt
| | Switch
--BATTERY---

Ideally some sort of guage with a good deal of 'swing' so I know when I am getting close.

regards

[horsefeathers]

my attempt at drawing using keyboard lines was a bag'o'shite!!

Looked good in the design stage!

regards

[twenty4/7]

what you want one of these http://cpc.farnell.com/jsp/search2/browse.jsp?N=500009+401+411+1003939&Ntk=gensearch_003&Ntt=split&Ntx=
from cpc.farnell.com order code cp02900 is the complete split charging kit or from most tow bar suppliers.
When the engine is off it disconects the van battery and your invertor runs on the secondary battery or keep
the engine running for hours of key cutting. I've had this fitted to my van for about 2 years with now no flat battery yet!

[WOT]

The current is 20 times as much on the 12v side and the voltage drop is 20 times as much.

To minimize this, you need the inverter as close as you can realistically get to the battery and avoid unnecessary connection junctions. Keep the 12v side cabling to absolute minimum and use an extension cord on 240v side as needed. If it's a big inverter, you may even need to mount it somewhere close to the battery, such as inside the fender.

Most inverter cuts out at 10.5v and if yours is shutting off at 11.5, there is voltage drop along the line.

[horsefeathers]

The current is 20 times as much on the 12v side and the voltage drop is 20 times as much.

To minimize this, you need the inverter as close as you can realistically get to the battery and avoid unnecessary connection junctions. Keep the 12v side cabling to absolute minimum and use an extension cord on 240v side as needed. If it's a big inverter, you may even need to mount it somewhere close to the battery, such as inside the fender.

Most inverter cuts out at 10.5v and if yours is shutting off at 11.5, there is voltage drop along the line.

eerrmm...maybe i didnt explain properly. I dont know at what voltage my invertor is shutting off, i only made up the figure of 11.5 for demonstration purposes. I am simply looking for a guage of some sort that when I initialise power (switch on the battery switch) a needle moves telling me the battery level. I can then monitor this each time I switch on and when I see the needle pointing to a certain position I know I need to get charging before long.

Thanks for all your thoughts though

regards

[ponsaloti]

horsefeathers, i have recently wired my second battery in the back of the van using a caravan charging kit, cost £12 and very simple distuctions. although fuse blew yesterday, as yet not sure why ?

[twenty4/7]

The caravan ones are quite low rated the car HiFi ones can run upto 180A. My own setup runs a 1000w inverter and 2 55w work lights from a 340Ahour battery more than enough power for me and allways ready to go.

[raimundo]

Im no sparky, but I would have thought of paralleling the battery with the autos battery to charge off the alternator, and using a switch to cut it from the parallel when cutting keys. Does that make sense to the sparkies?

[ponsaloti]

sorry, should have said my inverter is 1000w

[mark davis]

I got a great tip for you, throw away the 2nd battery and just run it off the engines battery, thats how mine operates never had a problem running machines and lights.
or run cables from engine battery to 2nd battery (trickle charge) that way no need to keep charging it.

[twenty4/7]

or run cables from engine battery to 2nd battery (trickle charge) that way no need to keep charging it.

that would be split charging setup without the switch add the relay to the red + wire and switch it from the ignition to automatically disconnect 2nd the battery

[mh]

Should I be looking for a voltmeter that can be placed 'in line' so to speak that always reads the battery voltage when the battery switch is enabled (would it use any power itself??), , and would it be worth it if 12v runs the invertor but say 11.5v fails to? The guage would hardly move between the two figures. Or should I be thinking in terms of amps and get an ammeter in line (as if I know what an amp is!!). Maybe amps would give a more accurate indication of battery level so that I could notice a greater swing between fully charged and <censored> close to failure!

So... Finally someone who appreciates electricians

The current flowing through the cables will not change much in your application (until the battery is drained to much, then it goes to 0... but that's too late then and you will notice anyway) - so an amp meter will not help.

The voltage is worth looking at, though.

Have a look at this: http://www.homepower.com/files/battvoltandsoc.pdf
The chart on page 3 shows that voltage is a good indicator of available capacity for lead acid batteries (I'm assuming that's what you have).

So you could connect a volt meter to each of the battery poles, maybe include a fuse if you want to avoid the hazard of a short.

As you said, analog volt meters don't show small differences very accurately, altough there are version for the car market that have a reasonable resolution.

I would go for a digital voltmeter instead.

Both do draw a tiny little bit of current / energy off the battery, but it's probably less than the battery is losing anyway on its own.

Hope this helps,
Cheers,
mh

[Buggs41]

Definately go with a digital voltmeter. It draws very low current from the battery due to its high input impedance.

An analog meter has low input impedance, and draws current from the source to show the voltage.

[fonzie123]

In my van i have a secondary 12v battery that i wire up to an invertor to give 240v in order to run my key cutitng machine. Works fine! (a 240v machine is more versatile than a 12v one in my opinion)

Usually i charge battery fairly regularly depending on how many keys I think I have cut since last charge. However, to remove guess work, i would like to incorprate something in the circuit (before the invertor) that could give me an indication of how much power the battery has left before it needs charging. Once or twice I have been merrily cutting away only for the invertor to squeal and the motor stops. The battery now needs charging before it will run the cutter again (obviously).

Should I be looking for a voltmeter that can be placed 'in line' so to speak that always reads the battery voltage when the battery switch is enabled (would it use any power itself??), , and would it be worth it if 12v runs the invertor but say 11.5v fails to? The guage would hardly move between the two figures. Or should I be thinking in terms of amps and get an ammeter in line (as if I know what an amp is!!). Maybe amps would give a more accurate indication of battery level so that I could notice a greater swing between fully charged and <censored> close to failure!
--------------
________________ ___ ______ ???? ________| |____
| _________ | | ______ ???? ________| invertor |___
| | | | ---------------
__________ -----
| | Batt
| | Switch
--BATTERY---

Ideally some sort of guage with a good deal of 'swing' so I know when I am getting close.

regards

the easyest way to do it go to a car audio shop what you want is an uprated alternator link the two batterys together and bingo as your driving both batterys charge at the same time u can use 1 battery for approx 50 mins on that system and still have charge to start and use the lights on your van fonzie123

[horsefeathers]

thanks for tips guys - after much deliberation I am going with the split charging system as mentioned by twenty4/7.

A few different ideas floated but that is what seperates us from the apes - free thought!!!

regards