site back-up battery gurus

[apco25]

Charged with specing and building out a battery back up system.

What I need to keep going.

4 Channel UHF Narrowband repeater system

3 Kenwood TK-750 x 40w 1.2a rx 13a tx
1 GR500 40w
Tx/RX preamp (part of combiner/multi-coupler) Negligble amp draw, but its 110v powered at this point and doesn't have a 12v input (I don't plan on tearing it apart to just get 12v to it.)

My idea was to set up a bank of batteries, East Penn 8G31DT managed by a durcomm BMS which provides charge, low voltage disconnect and isolation.

Should I figure the individual load or lump it all together? What duty cycle would give the best compromise? Duty cycle varies depending on the day of the week.

example

Using the RXvalue of 1.2a draw over 20 hours 24AH x 3 = 72AH need per repeater. 4 of the 8G31DT's would give me 384AH of run time.

Of course this will be wired in parallel with appriate gauge wire and disconnect switch.

Thinking of using an open rack at this point. Should I put in an inverter back to 110v to power the pre-amp? The way this system is designed its completely deaf without the pre-amp functioning so its a requirement to stay up with the rest of the sytem.

Guys, anything I'm missing?

[JohnWayne]

Check out the thread at http://batboard.batlabs.com/viewtopic.php?t=33225 for some good tidbits of info.

Regarding your varying duty cycle question, I would tend to plan for the worst case scenario. If your planned worst case is, say, transmitting 75% of the time, then calculate (DRAWtx*0.75 + DRAWidle*0.25) = amps per hour required.

I am not sure what the draw on a GR500 is, so check the manual. I would guess somewhere in the 12A range.

On the pre-amp, you will need to test to see if it is happy with a modified sine wave inverter. Otherwise you will need a true sine wave model. The draw on it will be fairly small, so just get a model that does a few hundred watts.

On the battery sizing calculations, don't forget that the capacity decreases as the draw increases. See the thread I linked to for all the nitty gritty details.

Jeff

[apco25]

OK I was looking for that post before and couldn't find it.

Using the calculation from the other post

.08 x .8 stand by current (normal squelched stand by)
.03 stand by current in battery mode
1.2 rx x .1
13 tx x .1

adding the results gets 2.06AH nominal with a maximum peak AH of 57.12 if everything is in use at once.

My guess giving our channel useage would be probably 15A draw with 1 repeater tx/rx and the others in stand by mode.

So I'm thinking I should bump up to a 125AH battery per repeater to get more breathing room?

[tvsjr]

Don't forget that, if you run an inverter, you're going to have a constant drain there, 24x7, of 100mA for a small unit.

Also, don't forget that the system is likely to become far more active if something bad happens, causing a power failure.

Do you have an idea of typical channel utilization on any given day? Also, how long do you want to keep the system running off battery?

[apco25]

This is not a 24hr system, fortunately.

The way it sits now Rptr 1 is potentially active from 0600 to 2300 hours with some weekends as late as 0200. Peak activity is weekends.

Rptr 2 active from 0600 to 1530 Mon-Fri with rare weekend use. Basically it sits in standby mode all weekend with a little car to car use if any.

Rptr 3 is active 0900-1700 Mon-Fri (with light channel loading) with peak activity on weeknds or special events (multi user channel loading)

Rptr 4 is active 0530-1700 7 days per week with light channel loading.

Figuring the duty cycle is a pain since its variable per channel per day of the week.

8-16 Hours was the batt run time I was shooting for.

[wa2zdy]

I hate recommending ARRL books but I just read through the ARRL book "Emergency Power For Radio Communications" and it was very informative. The author backs up all of his statements with references and real life experience and is very knowledgeable.

Anyone who needs emergency power NEEDS to have this book.

[RKG]

1. Load calc:

Go back to the cited thread. For EACH radio, separately calculate:

Squelched draw x 0.8, plus
Unsquelched draw x 0.1, plus
Xmit draw x 0.1.

Add the total for all radios.

Multiply that sum by 1.15 for inverter inefficiency.

2. Battery size:

The conservative approach is to take the result of the load calc and multiply it by 20. This will keep your average load within the 20-hr rating for the battery (or bank of batteries), and means that you can run for about 8-10 hours on batteries without violating the 50% depletion rule. No battery should ever be depleted beyond the 50% point; it will do unrecoverable damage.

The less conservative approach is to take the result of the load calc, multiply it by the max number of hours required to run on batteries, and multiple that by 2. However, this may violate the 5% rule.

3. System design:

MTRs need either 110VAC or the special 24VDC adapter provided by Motorola, which (in my opinion, at least) is not a good device and should be avoided. So you have two choices:

One involves having both an inverter and a charger, as separate components. Charger draws from the utility and provides DC to the batteries; batteries draw from the charger and provide DC to the inverter; inverter draws from the batteries and provide AC to the load. This is the definition of a true UPS, since there is no switching time and the load will never see a loss of power. However, it is the most expensive, both in terms of original cost and depletion of the life of the inverter, which is running 24/7. Also, the charger is somewhat specific and, in my experience, the only device I would recommend is TruCharge. Charger should be rated at not less than 10% and not more than 25% of nominal battery (bank) capacity.

My preferred approach uses a device called the Heart Interface, made by Xantrex. In one box, this combines a smart charger, an inverter, and a transfer switch. It has ports for AC in, AC out and DC (in and out). When there is utility power available, the Heart switches some of it directly to the AC load and uses some of it to charger or float the batteries. When AC is lost (defined as Voltage < 95 VAC RMS or > 135 VAC RMS or Freq <56 or >64 Hz), Heart switches to inverter mode and supplies AC from the batteries. Heart switches so fast (on the order of 20 msec) that no device that I've every hooked up to it--which includes PCs, Quantars and MTR2000s--sees the loss of power and reboots.

I've built a number of these power supplies for public safety radio and console installations. The primary purpose is to avoid the reboot while waiting for the generator to crank, settle and accept load or to carry everything when the generator fails to accept load long enough to get the Highway Dept. up to HQ with a skid-mount and a cord. Some have been in service for 5-10 years; to date, not a single failure.

In addition, this system is quite cost effective. A 1KW Heart plus 2 8G31DTs plus required wire, outlets, fuse and plastic conduit should be available for about $1,200, and can be assembled by someone who knows that he is doing in about 3-4 hours.

If you do a search on "Heart Interface" on this board, I believe you'll find a couple of threads that go into more detail.

[apco25]

Ok, I was up late reading the post last night so I missed a step in there.

I wasn't planning on running an inverter since the Kenwoods and the GR all offer 12v battery back-up input terminals. A small inverter will be used for the TX/RX systems pre-amp.

Looks like I'm on the money for AH rating and required equipment. I'l do some research on the heart device.

[M1225]

So all you need is a source of 12 volt and a small source of 120 for the pre amp? I say get a couple of good 6 volt deep cycle batteries(wired in series) a good small inverter and a iota power supply with the smartcharger option. some of the small 6 volt batteries offer up to 225 AH. With the Iota the batteries would stay maintained. You could run the inverter 24/7 of this setup but I dont know how long it would last. I seen those batteries made by crown that were 225 AH going for 96 a piece on ebay. Seems like a good price to me but I am not positive. This may not be the best way to do it but it would work.

[RKG]

I don't want to start (or rejuvenate) a debate, so I'll just say this and no more: I would not recommend the Iota chargers for any application.

[Bruce1807]

There are now fuel cells available for site back up. I was looking at them at the APCO show.
No maintenance required and last for years

[kj7xe]

There are now fuel cells available for site back up. I was looking at them at the APCO show.
No maintenance required and last for years

Yeah, we're looking at the methanol fuel cells for our fly-in single-radio mountaintop sites that get covered in snow/ice during the winter as additional backup to the solar/battery system. Looks very promising but quite expensive...

[apco25]

No doubt BIG money.

Don't really have a need for that here the site is easily accessible any time of the year.

I just need a good reliable system.

Cost proposal is in so just waiting on budget approval and I can start ordering the parts.

Thanks guys for the input!

[M1225]

Yeah I thought of another yet easier way to do this. My first way was not to well thought out. Tripp lite makes inverter chargers all you need is one of those and a bank of batteries. you plug the equitment into the unit and it plugs into an outlet. It has a built in battery charger and if the power fails it switches to the inverter. Only thing is you would have to size it to the equitment. Also takes care of the need for a low voltage disconect.

[RFdude]

Seems a shame that you have all this 12V equipment but cannot explore the same for the preamp. As you have pointed out, the RX preamp is a critical component without which you are deaf. For this reason, I'd put its source of power as close as possible to the battery without introducing additional failure scenarios.

In my experience, many of the RF preamps are internally fed with 12V. Cellular, VHF, UHF... many seem to use 12V at the actual amplifier. The AC-DC power supplies are very simple ones with a transformer, bridge, capacitor and small linear regulator. I haven't given it a moment of thought about converting such a receiver multicoupler to direct fed 12V albeit with appropriate fusing and EMI filtering. For the type of application you describe, Sinclair has a really neat RMC (although 24V input) with power monitoring. Since the LNA has two parallel amplifier stages, the power supply (drops the 24V to 12V with a linear regulator) incorporates a current bridge. Any drop or increase in preamp current from the norm will activate an alarm relay. Nice thing about a parallel stage LNA is that a hopeful failure mode is that we loose only one side, which will reduce our sensitivity, but not take it completely off the air.

To take your 12V up to 120V then back down to 12V seems an inefficient double convertsion. What about any potential effect of noise on the DC from the inverter? And you have two additional "blocks" that could fail and kill your system.

Good luck whatever you decide!

RF Dude.

[kb0nly]

I don't want to start (or rejuvenate) a debate, so I'll just say this and no more: I would not recommend the Iota chargers for any application.

Why?? I must have missed that debate. I got a Iota DLS-55 with the smart charging module plugged into it, this in turn is connected through a high amp breaker to a Optima deep cycle.

Best setup i ever had, rf quiet, no birdies in any of the radios, HF included, and give me all the shack DC i need. It's nice when the power goes out to keep the shack lit up with DC lighting and keep the radios running.

Don't see any reason to bad mouth Iota's, i know a lot of guys tickled pink with them.

[apco25]

From a simplicity (time and money) standpoint it just seems easier to work with the pre-amp as is. I do agree it is inefficient double conversion.

I'll have to find a time when I take the system down and look at the pre-amp without affecting the users.

[kb0nly]

So whats the deal on having the preamp anyway? Is the receiver that deaf? Is the antenna less than optimal?

Just curious..

[apco25]

The systems is 4 close spaced narrowband frequency pairs. The pre-amp is making up for the losses of the notch filters, TX combiner and RX multicoupler which was employed to utilize one antenna and feedline.

[RKG]

At the risk of beginning to sound like the Grinch in late December, stay away from the Tripp inverter/charger devices. (From memory, model numbers started with AP something.) These were tested a couple of years ago (as they are significantly less costly than other similar devices), and they failed to meet spec on several scores.

On chargers:

A true 3-stage charger (where "charger" means either a device to provide DC from utility AC or an alternator regulator) may be defined as follows:

Stage 1: Bulk:

Regulation: current (set at lesser of 95% of device capacity or 25% of battery nominal capacity at 20-hour rate);

Sense: battery terminal voltage; and

Trip point: when battery terminal voltage, adjusted for battery chemistry and battery temperature, reaches the "high point" (slightly below gassing for sealed-valve batteries and slightly above gassing point for flooded cells).

Stage 2: Acceptance:

Regulation: voltage (held at "high point");

Sense: current into battery (which is NOT the same thing as current out of the charger); and

Trip point: when current into the battery has decayed to 2% of the battery's nominal capacity at 20-hour rate.

Stage 3: Float:

Regulation: voltage (at the "low point," which is on the order of 13.7 (@70F) for SV and 13.4 (ditto) for flooded); and

Sense and Trip Point essentially N/A, since float mode runs indefinitely until charger reset.

In order to be a true 3-stage charger, a charger must be programmable for battery chemistry (or directly for high and low points) and have available inputs for charger output, battery inflow current, battery terminal voltage, and battery temperature.

Except in the case of complete battery management systems, you usually don't have access to battery inflow current, since this requires high current shunts on the individual battery negative lines. Some pretty good devices work around this limitation either by using a curve analysis to estimate house load (that current that is drawn from the charger but used to do other things, like carry fuel pumps and solenoids in the case of engine-driven sources, or radios, lights and other load, in the case of engine-driven or utility-driven sources) or simply use time as a surrogate. (Depending on ambient conditions, a setting of 0.5 to 2.0 hours is a reasonable surrogate for tripping the acceptance stage.)

However, any charger that is not programmable for battery chemistry, high and low setpoints, and battery capacity should be red-tagged.

A true float charger is one that is capable of holding voltage even when acceptance current is down in the weeds, that is, 50 mA or less into 1,000 AH nominal capacity. This requires some fairly expensive circuitry, and it is not common in most consumer-grade (and many commercial grade) standalone chargers. Rather, they default into a current regulated mode, which usually bottoms in the range of 150-500 mA. Over time, this will destroy a battery. Both the integral charger in the Heart Interface and the standalone Trucharge are capable of regulating float down to current levels below my capacity to measure (about 50 mA), which means that batteries can be left on them indefinitely without "cooking." There may be other devices out there with the same capability, but in 20+ years of doing this (which is about the life history of modern battery charging, if you put aside diesel/electric submarine applications, where it was done by hand) I've not encountered one. Coincidentally, both devices are sold by the same company and, more significantly, they are based on a design by the same two guys.