Jeep batterycurrent monitor--dual batteries

[qrper]

Boys and girls...

A winter project for sure, but getting the cats herded early.

Here's the poop.

One of my Jeeps has a dual battery system. Two batteries connected together (parallel) via a heavy duty relay.

I have one 0-50 amp current meter that is installed in the dash. A 50mv shunt is in the negative lead of the second battery. The meter displays the resultant voltage drop across the shunt.

I currently use the meter to monitor current draw from this second battery for secondary loads. But when this battery is being charged, the meter reflects downward, rendering the meter useless.

So, I am looking for a way to use the same meter and same shunt to display charging and discharge of this battery.

I know I can use a simple dpdt relay to reverse the meter leads to correct the meter's deflection, but would like a circuit to detect current going in one direction, then the other automatically. A LED (driven by the relay driver) would tell me what the meter is displaying i.e charging or discharging.

Being a bit greedy, could this same circuit be used to light up a second led warning me that excessive current draw is occurring?

Hall effect? SOunds good, but I've never used one before.

My thinking would be a op-amp reading the shunt, but I am not sure if the output would be positive (in ref to the ground ) or what nots?

There's a lot of sharp guys and gals here, so this one will be interesting.

thanks in advance..

mike

 

[gerty]

Easiest thing is to get a meter that displays 50-0-50 then you'll know which direction current is flowing.

 

[qrper]

Tis true!

However that being said, have you ever tried finding a center scale meter with a 50mv input? They're out there but one ups are very very expensive.

The old center scale meters of the past were moving vane types and did not require the shunt, but you had to run high current lines in and out of the dash. And to top it off, the older meters were really inaccurate. (think eminco (SP)meters)

An electronic solution would be more complex, but in the end, cheaper and easier to install. I don't want to recut a hole in the jeep dash.

 

[k7elp60]

You could still use a DPDT relay near the shunt, it doesn't have to be very big as the output voltage and current is very small. Use a DPST switch mounted near the dash to energize the relay. Use one set of contacts to energize the relay, use the other to energize a LED that comes on when charging. Wire the NC contacts of the relay for the polarity you want when the relay is deenergized.

 

[qrper]

That's doable... but automatically would be even better!~!

 

[on1aag]

However that being said, have you ever tried finding a center scale meter with a 50mv input? They're out there but one ups are very very expensive.

The one at the bottom right looks nice !
Pick two for five euro.

**broken link removed**

on1aag.

 

[qrper]

Do you have any idea how many minutes one of those would last in a jeep going off road? By the time I got to the trail head, the meter would be needless.

mike

 

[Willbe]

A 50mv shunt is in the negative lead of the second battery. The meter displays the resultant voltage drop across the shunt.

So it is a low-side shunt [one end of the shunt is grounded]?

P/L:

bandgap voltage references, 2 ea. with biasing resistors
IC comparator
12v DPDT relay with coil suppressor
LED + resistor
load dump transient filter to provide clean +12v

The +12v battery voltage pushes current through a biasing resistor through the voltage reference and to ground and the + reference terminal goes to the - comparator input terminal. The comparator - input terminal stays at Vref [4.096v or whatever].

+12v pushes current through a biasing resistor through the second voltage reference to the ungrounded end of the shunt, and the + reference terminal goes to the + comparator input terminal. The comparator + input terminal goes between Vref +50 mV and Vref - 50 mV, depending on the current flow direction.

When the shunt voltage goes through zero the comparator output switches from +12v to zero and powers or unpowers the relay which switches the polarity of the shunt voltage into the 50 mV meter. The LED indicates charge/discharge.

If the battery current is ~zero the LED might dither on and off, but I don't think this will commonly occur.

 

[gerty]

Check out #82202 **broken link removed**

You can get rid of the shunt and it's good for 60 amps.

 

[crutschow]

How about just using a cheap multimeter to measure the shunt voltage. They measure plus and minus and usually have a range of 199.9mV full scale which would give you a resolution of 0.1A with the 50mV, 50A shunt. You could power the 9V meter with a simple resistor zener shunt regulator from the 12V battery.

Of course you'd have to kludge it into the dash somehow.

 

[qrper]

that meter has an internal shunt. That's something I really as wanted to avoid if possible. The meter in the dash is the one I want to use.

 

[qrper]

yes, the shunt is in the grounded lead. From the battery neg lead to ground.

 

[mneary]

Internal shunt requires routing the entire +/- 60A to the dash. OP didn't want to do that.

 

[kchriste]

That's doable... but automatically would be even better!~!

You could simply power the DPDT relay coil from the "Output terminal" of the alternator, ignition circuit, or some other wire that has 12V when the engine is running. This method assumes that the alternator can keep up with any loads. If not, the meter will just read backwards as before.

 

[qrper]

At work tonight I had a "ah hah" moment.

this assumes one thing: when the battery is being charged, the current flows from the alternator to the battery. discharge the battery and the current flows the other way.

So... if one were to assume that charging means current flow positive, then no current flow or reverse current flow would mean discharge current.

So... All I would have to do is measure just forward current flow. Reverse or no flow would assume the circuit is discharging the battery.

Monitoring charging current would flip the relay to show + current. No charging current or discharge would NOT flip the relay and the meter would read correctly.

A led would be needed (across the relay coil) to note what is what.

So, I guess all I need is a way to monitor current flow and drive a relay from my 50mv shunt. What would be a good op-amp to use? More than likely I'll need a dual power source too.