need to draw 12v off 24 volt battery bank

[Mary B]

My inverter is 24 volt and that is also the best efficiency for my setup but my ham gear needs 13.6 volts at 40 amps max. Any ideas other than just pulling from half the bank (not an ideal situation for battery life). The dc/dc converters are to noisy receive wise to be used.

[Watt]

My inverter is 24 volt and that is also the best efficiency for my setup but my ham gear needs 13.6 volts at 40 amps max. Any ideas other than just pulling from half the bank (not an ideal situation for battery life). The dc/dc converters are to noisy receive wise to be used.

What about using a charge controller hooked to your 24v system which would charge a separate 12v battery.  You may then be able to use the power as needed, all provided from the charge controller? I'd also think the battery required for that 12v system could be pretty small to keep float losses to a minimum.  I wonder though, would a mppt controller be required for such an arrangement? 

[Mary B]

A separate battery won't be big enough to support 24 hour operation if needed.  Currently I have 520 watts of panels and will be buying 4 125 amp our marine batteries. More panels will be added as budget allows until I reach the max my MPPT-45 charge controller can handle.

[DamonHD]

Suppose you had a smallish battery being charged from the larger bank, primarily there to give you some smooth DC.  Possibly a PWM charger in 'slow' mode (a few Hz rather than a few hundred Hz PWM frequency)?  That will lose you some juice but with some huge electrolytics to dampen noise a bit more maybe it would get you there.

Could the intermediate battery be an Li-chemistry one to reduce energy loss?

Could it even be a super-cap being kept topped up by PWM from the main battery?

I have no idea how sensitive your radio gear is to injected noise on the power rails.

Rgds

Damon

[Mary B]

Very sensitive, I need to keep the noise to a minimum. Hope the charge controller is fairly quiet!

[dinges]

If efficiency is less important than noise, a custom linear regulator might be an option. Efficiency would be very terrible and a lot of heat would be developed, but you would be able to feed your 12V gear from your 24V battery bank. Far from ideal, but would work and be relatively simple to make.

If that's not an option (and if you're using precious, expensive RE then efficiency should be one of the most important things), I would go for a DC/DC PWM converter, either homebuilt or a commercial one. There are ways of making it quiet: shielding, grounding and filtering. Closed metal encasing, filter at the input, more filtering (filters completely shielded too), good bonding and grounding, lots of ferrites in the line, more filtering, feedthrough capacitors to keep things fully shielded when power enters another metal box in which the actual PWM sits. Then feedthrough capacitors to couple the power out of the box, more ferrites, more (shielded) low-pass filters. And a few electrolytics as well.

If you take a commercial PWM converter, build it in another (*completely*!) shielded and grounded metal box (and if you really want to go overboard, in another metal box), 60-70 dB suppression should be easy, 80-90 dB doable, 100-110 dB will be challenging but not impossible (likely involving a box-in-a-box).

There's no reason not to use PWM converters for noise reasons, as noise can be suppressed. The fact that most commercial equipment is lousily suppressed shouldn't deter you from adding your own shielding, IMO.

Not much more options, as I see it.... either tap the 24V battery at 12V, use a linear converter (insane), or use a properly built DC/DC PWM converter. Or get gear that works off 24V (or 28V). BTW, good radio equipment shouldn't be too sensitive to noise on the power lines.... it should have adequate filtering in the powerlines itself already. Noise picked up via antennas is a different matter of course, but that's where the shielding, grounding and filtering of a DC/DC PWM converter comes into play.

That's what I would do.

BTW, I assume most of the current is drawn by the final amplifier; it might be an idea to at least make that work off 24V. Transistors/FETs for those voltages (28V) usually have a little more gain and higher efficiency too than the 12V ones. With the bulk of power coming straight from 24V, the actual receiver/transmitter would probably only draw a few ampères at 12V and could be supplied with a much smaller linear regulator or DC/DC PWM converter (again, adequately shielded).

[vtpeaknik]

How about "pulling from half the bank" but switching between the two halves often?  "Often" could be every several minutes (no noise), or at a high frequency.  With fast switches (power FETs?) plus some filtering that may not add too much noise, and if done at a low frequency (every second say) the noise would not be a problem?

[ChrisOlson]

My inverter is 24 volt and that is also the best efficiency for my setup but my ham gear needs 13.6 volts at 40 amps max. Any ideas other than just pulling from half the bank (not an ideal situation for battery life). The dc/dc converters are to noisy receive wise to be used.

A friend of mine in Canada that runs radio gear and lives off-grid with a 24 volt system does it quite simply.  He got a separate 12 volt battery for the radio gear and charges it with an off-the shelf 12 volt battery charger plugged in to his 24 volt inverter.  The only "loss" is in heat in the battery charger, which is very, very small.  He's been running his ham radio station for better than 7 years with that setup, way up in the boonies north of MacDowell Lake.

If I could remember what his station letters are you could probably call him up there and talk to him about it.  He's on it all the time.  I know he has a 2 meter radio that doesn't reach out very far, and he talks to some other local people on that.  But he has another radio, that for an antenna, has nothing but a piece of wire stretched between two pine trees.  I don't know what meters that is, but he talks to people all over the world on it.

He calls me on his sat phone every now and then.  I'll ask him what his station letters are.

Balls, if you're pulling 40 amps you must have a 1 kW linear, like an old Kris Power Pump or something.
--
Chris

[dinges]

To show that switchmode PSUs can be quiet, here's a picture of a 48V batterycharger for the batterybanks  of our phonecompany (48V/100A out, 3 phase 400Vac input; I'm at the moment modifying it to single-phase 230Vac input and adjustable output voltage). As can be seen in the picture below, an entire shielded section of the 3HE 19" case is dedicated to filtering of the output:



(there is actually some more filtering in another compartment, not shown in that image)

And then there's the filtering of the input.... another dedicated shielded compartment, visible at the extreme right in the image below:



There are 3 filters in series per phase on the input side....

If noise-suppression is adequate for a phone company, then I'm sure noise suppression would be also good enough for amateurs... (when executed properly). As can be seen, quite different from the 'filtering' that's present (rather: lacking) in just about all consumer-grade switchmode powersupplies.... and they wonder why people complain about RFI?   

Here's a story to an amateur who designed and built his own 13.8V/40A switchmode powersupply, including a sub-section on noise suppression: http://ludens.cl/Electron/PS40/PS40.html With some changes in components (especially beefier primary switching transistors and turns-ratio of the transformer) it could be made to do what you require.

More information on the topic of noise suppression strategies can also be found in the ARRL handbook.

As others suggested, a separate 12V battery bank is another option. IMO hardly an elegant solution, as you then have two different systems and system voltages running at the same time, but it may well be the expedient option, depending on your requirements and skills.

[Mary B]

The extra battery is to much of a pain maintenance wise. 100 watts at 13.6 volts is ~8 amps with the transmitter stage being worst case 50% efficient the transmitter needs at least 20 amps by itself. The antenna tuners and other accessories all need power too and a little headroom is nice thus the 40 amp requirement. I will have to see what I can find for a dc/dc converter and try to tame it down.

[kitestrings]

We've had very good luck with these converters ('auot-transforemers' as they say), thought I don't operate ham gear.  We're currently using two on a 48V bank.

http://www.solarconverters.com/product_frame.html

I wonder if you just do a 12V tap and then balance the two halves - as opposed to putting the load directly on a converter - if you won't be okay.  Perhaps with some of the isolation measures that dinges expands on?

~ks

[ChrisOlson]

Anything you do to tap the bank, and try to keep it balanced, is going to be way more complicated, expensive and maintenance intensive than a separate battery to run the radio gear.

BTDT.  Didn't work.  When you find part of the bank at 14.5 volts getting deficit charged, and the other half at 15.5 getting the snot boiled out of it, you'll learn the hard (and expensive) way that tapping battery banks to get lower voltage power doesn't work.  Especially when you're talking about 1 kW intermittent load.
--
Chris

[dave ames]

Hihi MaryAlana,

Something of a ' when hobbies collide' situation here..the switching circuits that make our renewable energy equipment so nice leads to challenges in the radio side of the fun.

Wish there were a silver bullet to offer up for these interference problems..We (the radio club) run into this most every year during field day events.

Seems like a black art to me because the fix is seldom the same for any two stations. Along with the advice so far, shielding at the source of emissions, might try getting some distance between your RE power room and your radio shack/antenna. Also would try to mind that any pv array front is not lined up with any part of an antenna. (seems that the cell grid can do odd things).

Some large ferrite rings might help the pv line and rig power feed by passing both positive and negative conductors through several times as a choke. Could try some snap ons at our antenna element to coax junction..then trying different snap on beads and or rings with a turn count.,twisting/braiding the conductors..more shielding..other stuff like parallel caps as we clutch at straws..Good lord how I miss some of the 'Old Timers'.

Afraid we might find what works best for 10 meters might not do well at 80 meters.. probably won't have any trouble running FM 2 meters and some (seen a few..not me) talented CW operators can get through almost any amount of static.

A possibility if we do an inverter/grid to dc power supply..
http://www.ecrater.com/p/13320918/36-amp-12-volt-power-supply-cb

Best regards, Dave

KB1MZF

[kitestrings]

tapping battery banks to get lower voltage power doesn't work.

I'll be the first to admit that is not ideal, and, for this application there may be better ways.  I have no experience with ham radios.

That said, overall we haven't found tapping a battery bank to be a particuarly complicated, expensive or difficult.  I've found you do have to keep connection points clean and tight, and we periodically rotate batteries as well (I guess you could say that is added maintenance).  It probably works best when the loads are lighter, and variable, and where there is adequate 'off' time for balancing to equalize on regular basis.  It can, however, reduce the size of an equalizer capable of the doing the same job.

If I could have avoided I probably would, but sometimes when your system is growing or the end-use equipment is only availble in a 'unfriendly' voltages, the choices a limited.  Inverters at 48V are pretty common - not so much with things like CFLs, phones, answering machines, 12V do-dads, freezers.  And I still preferr this to realying on an inverter for everything.

~ks

[equiluxe]

Why not use a linear or analogue converter I used to use one in my Nisan patrol which was 24 volts to get 12 volts for ham gear.
I used an L200C as I remember it  and a large power transistor, had no problems operating it from the lighter socket.

[DamonHD]

Hi equiluxe,

Because as mentioned above it would be a scandalous waste of power (>50%) for a constrained renewables system.

The OP has to decide how much complexity and cost in the 12V system to trade off against power loss amongst other things I guess, but many of us would cry at the thought of throwing away at least half the expensively-generated energy!

Rgds

Damon

[equiluxe]

That's just it the wind is free once you have made the capital expenditure the power costs nothing, I built a system for my father some years ago and when the wind blows to hard the power is shunted to a 1000 watt fire bar. So the power loss in an analogue type dropper is small fry unless you have too small a set up. But what you could do is use a switch mode to drop to 2 or 3 volts above your target voltage then use a linear analogue unit to smooth and drop the rest of the way. That way your losses are minimal.

[DamonHD]

We know from other threads that the OP's budget and system size is constrained.

The two-stage (switch-mode and then analogue) approach is interesting.

Rgds

Damon

[Mary B]

Not running any wind yet, all solar so limited power budget. Adding a panel a month at ~$300 stretches my budget hard. Wonder if low resisitance fets could be used to isolate each half of the bank and parallel them. Advantage would be use as a power on/off switch also.

[equiluxe]

The two stage is not my idea, take a look at Dave Jones EEV Blog   http://www.eevblog.com/2012/03/20/eevblog-260-tracking-pre-regulator-simulation-in-ltspice-psu-part-13/

He goes into great detail on designing such a device there is about 14 hours of video on it to date.

[ghurd]

Someone posted a 'voltage converter modification' in the last few weeks, but i'll be danged if I can find it anywhere.

The efficiency is good, the adjustments were fairly simple, it can act as the controller for the 12V bank, and it has a 'master disable'.

The sketch in the link charges the 12V bank when the 24V bank has surplus power.
ghurd controller controls the disable function.
If the system is adaquite, the 24V side will charge the 24V bank instead of wasting thge power as heat.
http://i701.photobucket.com/albums/ww20/ghurd1/Sketches/ConvertrDisableCircuit.jpg


 

[ChrisOlson]

That said, overall we haven't found tapping a battery bank to be a particuarly complicated, expensive or difficult.  I've found you do have to keep connection points clean and tight, and we periodically rotate batteries as well (I guess you could say that is added maintenance).  It probably works best when the loads are lighter, and variable, and where there is adequate 'off' time for balancing to equalize on regular basis.

Tapping the bank is not complicated.  Keeping it balanced is.  Nor is tapping the bank expensive.  Buying new batteries because they didn't stay balanced is.

I tried tapping my bank to keep my genset battery charged due to the generator not being used for months during the summer, and the electronics in the generator draw a few milliamps so the battery goes dead after awhile.  The genset battery is just a little 12ah SLA.  That tiny battery hooked to one of my big ones in the bank caused the big one to run .25 volts below nominal and the one it's in series with to run .25 volts above nominal.

This causes the one that runs low on voltage to sulfate.  It causes the one that runs high to have shortened plate life.  You cannot do that to batteries and get full life from them.  Period.

MaryAlana is like us - off-grid.  When you live off-grid there is no such thing as resting batteries to let things "equalize out".  Those batteries are either being charged or discharged, 24 hours a day, 365 days a year.

Tapping the bank and rotating batteries is not an option for off-grid folks.  For people with hobby systems that might work ok because they like to work on their system all the time.  For folks who depend on the system for their daily power it has to be done right or you're buying batteries all the time because they don't last.  And that's the expensive part of tapping a series battery bank.
--
Chris

[Mary B]

A single battery won't be enough to run the ham station, plus it adds quite a bit of cost and headache. I know I could tap the batteries with diodes and live with the voltage drop so that is why I was thinking of fets, the voltage drop is minimal.

[kitestrings]

That tiny battery hooked to one of my big ones in the bank caused the big one to run .25 volts below nominal and the one it's in series with to run .25 volts above nominal.

?? Chris, I'm curious was that with an equalizer/balancer in use?

When you live off-grid there is no such thing as resting batteries to let things "equalize out".  Those batteries are either being charged or discharged, 24 hours a day, 365 days a year.

Well, the equalizer we use works to buck or boost (a misnomer, but hey they call it a 'autotransformer') and your right it almost always working 24-7, but load and the generation varies tremendously.  It does in fact catch up in fairly short order when one, or the other ramps up or down.

Tapping the bank and rotating batteries is not an option for off-grid folks...or you're buying batteries all the time

I rotate the 4 (overall 2-series strings, 48V) serving the 12V loads about every 1.5-2 years, at the same time recording SG, cleaning connections, topping water levels, etc.  Our last set of batteries lasted 10-years.  Soley off-grid.  YMMV.

~ks

[ChrisOlson]

?? Chris, I'm curious was that with an equalizer/balancer in use?

No, I was using a Morningstar RD-1 to turn on power to the genset battery when the voltage of the bank got up to where it could keep it charged.  I didn't think that little 12ah battery could cause any problems.  I was wrong.

I suppose it's possible to come up with some sort of arrangement of electronic wizardry to keep a 24 volt bank balanced with a 12 volt load on it and no DC->DC converter.  If you want to live the complexity of it, that is, and you still have to keep a close eye on that bank to make sure it's working right.  Or you'll ruin your batteries.

I think it was MaryAlana that commented earlier that a single battery could not run the ham gear.  I tend to disagree on that.  If you're just using a single Group 29 marine deep cycle it would have to be kept on constant charge.  But a Group 29 has no problem delivering 40 amps intermittent with a constant charge to it for years.

Some people look at things differently.  I have 24 200ah batteries in my off-grid bank and they are not a headache, nor are they high maintenance.  I service them four times a year.  My lawn is much more of a headache and higher maintenance than my battery bank.  My dang lawn cuts into my fishin' time.  My battery bank doesn't.
--
Chris

[kitestrings]

I don't think she mentioned the configuration of the bank.  If it were say two series strings, one could do a center tap, but parallel the tap also...

I'm probably not explaining it well...but say you had 4- 12V batteries to make it simple; two series strings (24V), in parallel.  You could do a center tap, but parallel the tap, so you'd have two batteries (at 12V) working for you.  You'd still need an equalizer/balancer, but you could every so often swap which half of the bank you pulled off without moving anything (other than wires).  If you'd did, you'd want to keep the (12V) -neg independent of -neg relative to the 24V bank.

Of course this doesn't solve any noise issue that you might have with the converter, and it certainly doesn't free up any more time for fishin'

~ks

[ChrisOlson]

IIRC from another thread I think the bank configuration for MaryAlana is either two or four Group 29 marine deep cycles from Walmart in a 24 volt configuration.

The bigger the battery, the less tendency to get out of balance when you have unequal loads on the halves of the bank.  Those Group 29's are pretty small so I'd be leary of anything that would put an imbalance on them and swap batteries to even it out.

What happens is that on a good solar day the bank gets up to absorb pretty fast.  You got an imbalance so one battery runs low and the other one runs high.  So the battery rotating method swaps that around and the next day the other battery low and the first one runs high.

At that point, you already shortened your battery bank life, regardless of rotating which battery has the imbalanced load on it.

If you have insufficient incoming power to get the bank to absorb voltage, then you won't boil the snot out of one battery.  But the real problem I see with it is the fact that those little Group 29's won't handle that sort of abuse.  They will serve you long and well if charged properly every cycle.  But abuse them by running them high or low on voltage - by even .2 volts - and they will die an early death.  They just don't have the plates in them that a big RE battery has.
--
Chris

[Mary B]

Found a source for 232AH (20hr) 6 volt batteries for $100 each. 2 came home with me today, pick up 2 more next month so that should cover the batteries for the system.

[ChrisOlson]

Being you (evidently) don't have the system set up yet why don't you just go with 12 volt and eliminate the problem?

I'm so religious about making sure my bank stays balanced I got volt meters installed on the low voltage taps on the bank to monitor it.



I've always been an Analog Man and never could get in "tune" with digital meters.  With analog when all the needles are laying on their pegs you know you got power (or she's about to blow).   
--
Chris

[Mary B]

At 24 volts the charge controller can handle 1200 watts of panels, at 12 only 600 watts. I want expansion capability.

[ChrisOlson]

At 24 volts the charge controller can handle 1200 watts of panels, at 12 only 600 watts. I want expansion capability.

Well, let's be realistic.  All it takes for future expansion capability is to swap a couple wires around on the bank when you get to 600+ watts of installed solar capacity.  In the mean time, if you are running ham gear off-grid and need 12 volt power to do it, guess what the logical choice is for system voltage?

I mention this because it doesn't sound like you got your system set up and operating yet.  Putting in a 24 volt system when your main load is 12 volt is putting the cart before the horse.  And I also mention that because I know that 600 watts of installed solar capacity is not going to go far in running an amateur radio station 24/7.

If you haven't bought an inverter yet, I'd be strongly considering a 12 volt system, simply because the major power consumer in your system is 12 volt.
--
Chris

[Mary B]

Should be fine for running 24 hours occasionally. Transmit time is typically 25% or less so a max if 1600 watts based on 20 amps is needed. I do have a 12 volt inverter that gets used in the car on occasion so I can run that for now but by the end of the summer I should have 4 more 140 watt panels to install. I will have 464 amp hour capacity at 12 volts.

[ChrisOlson]

For a given bank of batteries, the kWh you can store is the same regardless of system voltage.  If you have the four 232 ah 6V batteries, you got 464 ah @ 12 volt and 232 ah @ 24 volt.  However, the kWh is the same at 2.78 with either setup.

It just makes more sense to me to stick with the 12 volt for now.  If you add panels and get the limit of your solar controller, then you have a decision to make:

• drop the bux on a new 24 volt inverter
• come up with a way to get 12 volts for your ham gear

OR:
simply buy an additional solar controller for your solar to handle the amps.
--
Chris