Radio-in-a-box engineering issue

[Kainroh]

A little background first...

We've been tasked with designing and building a number of "Radio-in-a-box" for use at small command posts during earthquakes, tsunami, disasters, etc. It's purpose will be to provide communication to "Urban Rescue" personel on portables. Basically these will be a mobile VHF radio, mounted in a pelican case with a battery and charging system. Typically these will be powered by a generator, but *may* need to be used on battery alone for short periods of time. This is to replace the portables (HT1000) that they currently use, providing a higher power alternative. I'll add that this is an in-house project, not a product that will be offerred for sale.

We have sourced the following for this project:

Motorola XTL1500
Pulsetech Charger http://www.pulsetech.net/XC100P-Xtreme- ... -7346.aspx
Discover 35Ah Battery http://www.discover-energy.com/sites/de ... 12350D.pdf
Pelican 1440 Case (Orange) http://www.pelican.com/cases_detail.php?Case=1440

Now here's our problem.
With charger, battery and radio all connected, keying the radio draws too much current through the charger, sending it into an over-current protection mode. It recovers fairly gracefully, but I'm not confident it could do this repetatively.

What we are considering is installing a relay into the circuit to disconnect the charger when the radio is keyed. While I know there is an PTT input on the XTL1500, what we need is a PTT output. One option is to forgo the front panel mic connection and instead use the AUX MIC (pin23) and PTT (pin16), which we could also use to actuate the relay.

I've also considered putting a relay on the microphone hook, so that only when the mic was hooked would the battery charge. Not as elegant as a PTT switch, but workable if no other option exists.

Should we pursue a larger charging system with greater current handling capability instead?

Anyone have experience designing something similiar?

[RFguy]

Use a different power supply/charger.

The one you have has no low-voltage disconnect. They will leave the radio on with the AC power disconnected and drain the battery to zero, significantly reducing battery life.

We build the same thing with 2 x 7.2 AH batteries, power supply and a radio in a Pelican case. We have a special bracket made to hold the batteries on top the power supply.

We use the ICT battery charger series power supplies. It properly regulates battery charge and has LVD built in.

http://www.ict-power.com/products/battery_chargers/
http://www.ict-power.com/products/displ ... hp?id=1013

Relays connected to PTT switches/hang-up boxes just seems too Rube Goldberg.

[Astro Spectra]

Are you certain it's not RF getting into the charger? Maybe test with a dummy load.

[RFguy]

It's only a 2.5 amp charger trying to supply a 8+ amp draw (assuming a 25-watt radio).

Running the under-sized unit in regular overload is not a great idea.

[Kainroh]

It's only a 2.5 amp charger trying to supply a 8+ amp draw (assuming a 25-watt radio).

Running the under-sized unit in regular overload is not a great idea.

Our idea was that the battery can supply plenty of current, the charger is simply topping it up between periods of high load (transmitting). A duty cycle of perhaps 10%.

This might be optimistic though, that`s why I ask.

[wavetar]

I second the ICT power supply suggestion. Great units and it will eliminate your problem, as it is the correct equipment for your purpose.

[Jim202]

Sounds like a disaster that has already happened. One of the problems that all engineers face is the design specs of what ever they are doing or working on. In this case, your asking the impossible from a power supply that will never work. You can't expect the battery to hold the voltage up during transmit if the power supply can't pull it's weight.

I would suggest that you go do some reading on how your battery will act with the 8 amps or so load put on it during transmit. The next check you need to do is to leave the power supply off and see just where you go with the battery terminal voltage when you key the radio. Bear in mind that the radio has voltage regulators and voltage sensing circuits in it. The micro will be put into a reset if the supply voltage goes too lo. I am not sure just what the trip point is on the XTL1500, but you need to find out just what that point is.

Once you have a handle on the low voltage trip point where the radio will roll over when you transmit, do a test. See how well your battery terminal voltage holds. Determine what ever duty cycle you think might be worst case use of your package. Once you have some idea of what the parameters are for your operation, then you can see if your choice of battery will survive.

After the battery test, then go back and look again at what your going to need for a power supply. You might consider using a diode isolation if the power supply tends to become a problem. You can always raise up the power supply output voltage to compensate for the diode drop. Remember that if the battery is down any, your going to need extra current to bring it back up to the max charge the battery can tolerate. Don't forget that the battery will have a max charge rate. You also need to take this into account. It might even require to add a low value resistor to limit the max charge rate going to the battery.

Such are the issues that face any engineer with a project they take on. Let us know how well your making out on your homework on this one.

Jim

[Bill_G]

I'd use a more economical model of radio. Reduce the cost of the radio and use some of the savings to increase your battery and charger capacity.

[KE7JFF]

Oh, I've built plenty of these radio in a box type solutions!

What I like to use is an 12V 2 Amp battery charger (the kind that looks like wall-wart but has its own AC coord) from an auto parts store. The one I use with my box will shut off when a load is put on the battery, which is perfect. I can't think of the brand off the top of my head, but most of them in the 2 amp category I've seen have this feature.

[fineshot1]

I would go with this supply/charger as it can supply both needs at the same time when charging
is needed most and automaticly. You should probably use a larger pelican case if need be due to
the IOTA unit being larger and outboard a Optima Yellow Top marine battery. This combo has Never
failed for me and is robust which is what you need for public safety usage.

http://www.iotaengineering.com/dls15.htm

[RFguy]

I would go with this supply/charger as it can supply both needs at the same time when charging
is needed most and automaticly.
http://www.iotaengineering.com/dls15.htm

It doesn't look like that unit has a low voltage disconnect. We have found LVD to be imperative in Pelican cases as users often leave the radios turned on after use. Without the LVD, battery life is significantly shortened.

LVD can be added as an external unit, but is easier to use a unit such as the ICT12012-12BC that has LVD built in.

On the side: ICT has a unit to add-on battery back-up to an existing power supply. It has battery charging, battery revert and LVD built in at a reasonable price.
Model is a ICT12-30BM http://www.ict-power.com/products/displ ... php?id=822

[fineshot1]

I would go with this supply/charger as it can supply both needs at the same time when charging
is needed most and automaticly.
http://www.iotaengineering.com/dls15.htm

It doesn't look like that unit has a low voltage disconnect. We have found LVD to be imperative in Pelican cases as users often leave the radios turned on after use. Without the LVD, battery life is significantly shortened.

LVD can be added as an external unit, but is easier to use a unit such as the ICT12012-12BC that has LVD built in.

On the side: ICT has a unit to add-on battery back-up to an existing power supply. It has battery charging, battery revert and LVD built in at a reasonable price.
Model is a ICT12-30BM http://www.ict-power.com/products/displ ... php?id=822

LVD is an option that is mostly used on base station equipment and if your going to try and squeeze in
every option you can for base stations you are going to find it most difficult to squeeze in and increase
the cost Dramatically. I suggest you compromise your goals to keep size and costs down.

Any public safety user of an emergency mobile radio system such as this has to be aware of the useage
and cautions of such a unit.