Battery monitor, cheap and easy.

castaway

castaway

Well-Known Member
Joined
31 Dec 2001
Messages
1,573
Location
Solent
uk.groups.yahoo.com
Im always playing 'catch up' with my battery power, but being un willing to pay out for a plumbed in system usually end up clipping my 5 quid Maplin multimeter onto the battery terminals, to check which of the 2 banks is least flat.

Whilst away this week it occured to me that, a far more sensible way of achieving the same result was to snip off the stylus/prong things at the end of each lead on the multimeter and fit a fag lighter output plug ( which I had lying around). I thnk most people have a fag lighter socket o/board these days to keep mobile phones etc. charged?

Now I can continuously monitor my battery state by plugging into the fag socket and the multi meter just sitts on the corner of the chart table. All for an additional cost of absolutely nothing..

Amazing what you come up with on a weeks sailing with no wind and continuous drizzle....

Nick
 
Great idea. I tried a less plumbed in method by using the prongs on the cig lighter socket. The only trouble was that the voltage loss caused by the thinish wires to the cig socket meant I got an inaccurate reading. And the reading was meaningless when the wind genny was going. Friends boat has a similar set up to yours but he hard wired the voltage meter to the battery terminals and stuck the meter next to the battery box.
 
Yes I previously had mine wired (with crock clips) to the bat terminals, except I had to swap'em over or have 2 meters for the 2 banks. Also I found that the crock clips tended to disintegrate rather quickly.

I must do a check to see if there is any voltage drop between the multimeter direct to the battery and taking the reading from the fag socket.

Nick
 
Most cheap multimeters have an impedance (resistance) on the DC Voltage readings setting of greater than 1Million Ohm, so it is unlikely that it would draw enough current to cause a measurable voltage drop on even the thinnest of cables, assuming nothing else is drawing current on that cable. I have a handy chart next to my voltmeter that tells me the state of the battery, but as mentioned when solar/wind chargers are connected it's no use.
I try to keep the batteries between 50 & 80% when mains is not available. The voltages are measured with a small load on the batteries.

100% - 12.73V
90% - 12.62V
80% - 12.50V
70% - 12.37V
60% - 12.24V
50% - 12.10V
==============
40% - 11.96V
30% - 11.81V
20% - 11.66V
10% - 11.51V
 
[ QUOTE ]
The only trouble was that the voltage loss caused by the thinish wires to the cig socket meant I got an inaccurate reading

[/ QUOTE ] The current drawn by a digital multimeter is miniscule so even the thinnest of wires will not cause any voltage drop if it is just the meter connected to the circuit.

I hope your cig lighter circuit has a fuse included the value of which reflects the small size of the wiring!

The only meaniful voltage reading if you are interested in state of charge is one taken with no load or charging and taken after the battery has rested for at least 12 hour after the last bout of charging. To make comparable readings always wait the same length of time.

Very roughly a good fully charged battery will read 12.7v, 12.8 if you are lucky. You can take 11.7v as effectively flat and assume a linear relationship over that 1.0 volts difference with state of charge. Thus 12.2v is about half charged.

Personally I would prefer to monitor at the battery terminals or with a (fused) dedicated circuit from there.
 
I have a nasa battery monitor that shows lots of useful info like the time to full charge or discharge, amp hours. It also takes account of battery temperature which can effect the available capacity. Plus the unit is waterproof and backlit so can be fitted outside on a display panel and seen in the dark up on deck which is useful for sailing at night. It is continually connected and uses very little power. For £90 I thought it was well worth it.
 
A real battery monitor like the nasa BM1 or the LINK measures the battery voltage and the current and caculates the state of charge.
While knowing the voltage may be useful it cannot give you the state of charge without knowing what load was on the battery when the voltage was measured.
 
I use a car type thermometer / clock with a voltage digital read out
radio shack do a good one.Hard wired with fuse from each battery with a 3 position switch 1 2 and off.tells you what the charge rate is as well.c an always tell whats happening with the charging system.
 
I spent ages trying to find a voltmeter that was reasonably priced. I honestly can't figure out why you cant buy one for the same sort of price as a multimeter, all I could find was amp meters/counters with shunts that go for £100 or more.

I eventually opted to just plumb in the analogue volt meter that I had already, and then obviously spent the whole season using a multimeter anyway because you cant read sh%&t off an analogue one.

I then found these on ebay. It is absolutely the best value bit of kit I've found, the guy's ebay shop does loads of voltmeters, led, lcd, backlit. Highly highly recommend them.
 
Various ways ...

I have my multi-meter ready for surgery next time back ... it's an analogue jobbie that basically the rotary switch part has failed ... so solder wires onto internal contacts fixing it as a Voltmeter .... wires going to a switch so that I can decide which battery I want to check.
At present - I have battery voltage displayed on the plotter - and that can be used via the direct to domestic battery or via the 1-both-2 -off switch - so getting a reading of either battery.

The ciggy lighter socket is one I hadn't thought of ....

One item that I want to also fit - I have a 9.99 Halfords LED Alternator / Battery checker .... cheap but effective battery checker for charging etc. I want to fit via a press to make set-up for checking alternator output and also batterys .... (Maplins now do them for about a fiver I think ????)
 
E-Bay etc. .....

Don't doubt it .... I already have the bits and my GPS Plotter gives actually a very good indication of battery Voltage ... in fact as things switch on / off - it shows literally straight away fluctuations of voltage as demand changes .....

Other day on my home bench - used it to follow an old 12V car battery down its discharge .... checking with separate digital multi-meter same time .... spot on.
 
I thought about that solution but was eventually put off because that requires a separate battery for the DMM and that cannot easily be wired into the battery it is monitoring.

Instead I found a very simple 2 wire meter from RS: meter that is very simple to connect - via a SPDT(Centre off) switch to either bank.
 
Interesting that they now say "The meter can be powered with the SAME SOURCE being measured". I found that didn't work, the reading hunted wildly. It works if you filter the supply separately from the sensing connectiions - I used a 5uF capacitor and a couple of diodes, but it slows the switch-on time. I wonder if their product has changed since I got mine.

Still not fitted mine, but the intention is to fit with two push-to-test switches to check either battery. (Obviously wired as interlocks so pressing both doesn't connect the two batteries together)
 
Surely a cheap £5 meter ....

Such as the little rotary switch jobs like I have are good enough with a push to make switch etc. for connection ? take it apart ... solder leads to respective contacts ... leave of the rotary switch bit ... then the display part is normally a few mm proud - making it easy to mount into a nice wood plinth ..

No need for diodes, fancy bibs and bobs ....
 
Re: Surely a cheap £5 meter ....

If I get the time to neatly fit my panel meter, that'll definitely be the smartest built-in solution, and not really any more expense either. However I'll probably have a cheapy meter onboard anyway, so there ought to be some way of making use of that instead.

How will you deal with the battery on the multimeter, rig the push-to-test so that it switches on the battery as well as connecting the load? The trouble is that it starts to get as much trouble as mounting a purpose-made panel meter, and also stops the multi-meter being available as a general purpose tool.
 
You must log in or register to reply here.

Share this page

Top