Author Topic: Davis Battery backup  (Read 10477 times)

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Davis Battery backup
« on: March 26, 2011, 07:08:40 PM »
Ok so how can the best use be made of the Davis battery backup especially with regard WLIP and in remote systems?

The Envoy 3*AA and Console 3*C are pretty much useless when running a WLIP providing only a few hours.

So how can we make use of the battery backup curcuit to improve backup times? Thoughts?


One thought I have had is to extend the battery backup +/- wires from the circuit board (very easy with the envoy) and run external backup but what voltage will this curcuit handle and how does this curcuit interconnect with the primary external power input?

Typical power spec is rated at 5 volts and we know the primary cuircuit can handle 6 volts (even up to 7 volts). Battery backup is typically 4.7-4.8 volts with new batteries so the scenario might be

Primary 5 - 7 volts, backup 4.7 volts (typical)


What if it was

Primary 6 volts, backup 6 volts?
Primary 5 volts, backup 6 volts?

Thinking more of trying to eliminate DC/DC convertors (like 6 to 4.5v or 12 to 4.5v) from the system as this only introduces something else in the system that could fail and run direct from battery. I have seen 4.5v SLA batteries but what about a charger?   

Offline johnd

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Re: Davis Battery backup
« Reply #1 on: March 27, 2011, 07:23:31 AM »
The Envoy 3*AA and Console 3*C are pretty much useless when running a WLIP providing only a few hours.
I'm not convinced that the WLIP will run reliably on battery power at all. Whenever I've had the power go down on a WLIP system, it's always required a power recycle to reset the logger even though the Envoy/console had batteries in. Maybe I just wasn't lucky enough to catch it in the first few hours of outage or maybe the consoles batteries were already partially depeted - must admit that I haven't run any rigorous tests of WLIP on battery power alone. But I do also wonder whether the WLIP actually does benefit from being powered from 5v and not the 4.0-4.5v of a partially depleted battery.

But maybe you have done some tests?

Quote
So how can we make use of the battery backup curcuit to improve backup times? Thoughts?
I'd suggest that the only simple solution is to wire the battery contacts to an external battery box using primary cells. If you had a 6-cell box of D cells wired as two parallel banks of 3 cells then that would give you a respectable current reserve. (But this is obviously on the assumption that 4.5v is indeed sufficient to power the WLIP reliably.) BTW I'm not sure how you can have a 4.5v SLA battery: two LA cells gives you a voltage of around 4.0v (maybe 4.2v @ literally 100% charge and no/minimal load) and three LA cells would be 6v nominal. Are you sure it wasn't say a 4-cell nicad pack that might give you eg 4.8v?

Also the max recommended voltage to apply to a VP2 console is 6v (personal communication from Davis).

Quote
Thinking more of trying to eliminate DC/DC convertors (like 6 to 4.5v or 12 to 4.5v) from the system as this only introduces something else in the system that could fail and run direct from battery.
Hmm, I'm not sure that as soon as you move from a simple external battery box (which could I guess be an offsite-rechargeable pack in principle) then any of the solutions is swapping one type of added complexity for another.

For example, it is possible to imagine the console being powered from an efficient 5v or 6v UPS that is fed 12v DC from a solar PSU. I think the circuitry for this sort of device is fairly straightforward and it could use any size of 6v SLA that you cared to specify. But I haven't come across anything like this that is readily commercially available (I dare say that they do exist but where do you get one from and at a reasonable price?)

Obviously, another option is to use a separate 6v solar panel/controller/SLA just to power the console. Although 6v components are not as easy to track down as 12/24v, you can with a little effort find 5W 6v panels and regulators, which are potentially powerful enough to be useful if they're just powering  the console/WLIP. (At some point in the design process you have to bite the bullet and decide just how much current/power reserve you want to build in. So a 17Ah 6v battery, to take a size at random, would give you around 170 hours - roughly a week - WLIP run-time. )

But I'm not convinced that putting in eg a separate 6v PSU for the console/WLIP or using a 6v PSU (if you can find a suitable part) is any simpler than the solution I use which is to reserve the last 20% or so of the capacity of a 12v/100Ah battery purely for use by the console/WLIP. Yes there's an electronic voltage switch involved and also a 5v DCDC converter, but both parts seem pretty reliable and are relatively inexpensive to buy.
Prodata Weather Systems
Prodata's dedicated Davis EnviroMonitor website
UK Davis Premier Dealer - All Davis stations, accessories and spares
Littleport, Ely, Cambs UK

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Re: Davis Battery backup
« Reply #2 on: March 27, 2011, 08:15:09 AM »
Quote
Are you sure it wasn't say a 4-cell nicad pack that might give you eg 4.8v?

You're right the 4.5 figure is A/h of a 2*2volt battery bank giving 4.8v

Quote
Also the max recommended voltage to apply to a VP2 console is 6v (personal communication from Davis).

This then differs from what the Davis solar panels can put out and also the float charge of most 6 volt staged/smart chargers connected to a 6 volt battery, which is around 6.8 volts. 

Quote
But I'm not convinced that putting in eg a separate 6v PSU for the console/WLIP or using a 6v PSU (if you can find a suitable part) is any simpler than the solution I use which is to reserve the last 20% or so of the capacity of a 12v/100Ah battery purely for use by the console/WLIP. Yes there's an electronic voltage switch involved and also a 5v DCDC converter, but both parts seem pretty reliable and are relatively inexpensive to buy.

So I assume then you consider the battery backup circuit as redundant?

And maybe this is where I see an issue? Here we have a reasonably low power 5-6 volt system yet we have to use quite large Amp/h 12 volt systems and associated add-on curcuits to keep it running?


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Re: Davis Battery backup
« Reply #3 on: March 27, 2011, 08:23:50 AM »
Ok so this raises another query.

Variable input DC/DC convertor capable of outputing 5 volts with either an input of 6 or 12 volts.

Would the same A/h 6 volt or 12 volt battery provide 5 volt output for the same amount of time? 

Offline johnd

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Re: Davis Battery backup
« Reply #4 on: March 27, 2011, 10:09:00 AM »
This then differs from what the Davis solar panels can put out and also the float charge of most 6 volt staged/smart chargers connected to a 6 volt battery, which is around 6.8 volts.

Two comments: First I've never really taken a look at how sophisticated the regulator on eg a 7707 Davis solar PSU is. But if it was more than just a trivially simple controller (which possibly/probably isn't the case) then the load output might be a regulated 5-6v value.

Second, I'm sure that 6v isn't a hard limit in the sense that 6.1v will cause the unit to fail. It's more that the console PCBA components will be rated for a max continuous voltage of around 6v. There will obviously be some margin on this and so maybe 6.8v for perhaps 10-20% of the time might well be perfectly acceptable, though it might be on the margin of causing slightly premature ageing. But plugging in say a 7.5v or 9v mains supply for permanent use certainly wouldn't be advisable.

Quote
I assume then you consider the battery backup circuit as redundant?
Possibly, but I'm really not sure. All I can say is that IME the battery backup doesn't seem to work too well with the WLIP logger installed, but I haven't done any rigorous tests.

Quote
And maybe this is where I see an issue? Here we have a reasonably low power 5-6 volt system yet we have to use quite large Amp/h 12 volt systems and associated add-on curcuits to keep it running?

I'm not sure why this would be an issue. The power density of batteries is simply not very good at delivering the levels of current that we take for granted from a small mains adapter over an extended period. I have toyed with the idea of using a methanol fuel cell like this one:

http://www.efoy.com/en/fuel-cells-technology.html

But these are still very expensive to buy. Maybe in a couple of years time? (Though Toshiba - IIRC the relevant make - have been rumoured to be about to bring one out to power laptops for several years now that might well be substantially cheaper. It's obviously close to production but seemingly not quite close enough as yet for whatever reason. Or maybe early production has started but availability is only in Japan? Certainly AFAIK they're not readily available outside of Japan.)

Part of the problem seems to be that TCP/IP connections (ie as used by the WLIP) seem to need to use a substantial chunk of power. I don't understand enough about network design & engineering to know why this should be the case. We wouldn't be having the same discussion if all we wanted to do was to power a VP2 console with a serial logger say - this is why I say (as per my post in the previous thread) that the VC sort of architecture is probably the best if you want to consume the absolute minimum of power at a remote site.
Prodata Weather Systems
Prodata's dedicated Davis EnviroMonitor website
UK Davis Premier Dealer - All Davis stations, accessories and spares
Littleport, Ely, Cambs UK

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Re: Davis Battery backup
« Reply #5 on: March 27, 2011, 10:37:36 PM »
As an aside I have been making enquires re 5v, 6v & 12 volt options. 5 volt AC/DC appears readily available, 6 volt from many suppliers is a special, anything in 12 volt no problems at all. 

6 volts appears to be out of flavour especially re small profile / low wattage battery chargers. Spoke to one supplier this morning who requires a minimum 100pc order? 

Offline johnd

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Re: Davis Battery backup
« Reply #6 on: March 28, 2011, 06:17:11 AM »
As an aside I have been making enquires re 5v, 6v & 12 volt options. 5 volt AC/DC appears readily available, 6 volt from many suppliers is a special, anything in 12 volt no problems at all. 

Sorry, I'm probably not awake yet, but I'm not clear exactly which part you're referring to here. I think that it's battery chargers that run from AC mains, but 6v chargers are AFAICS readily available. Is it maybe a charge regulator/controller or a DC/DC  adapter or maybe a 6v battery charger that wil run from a higher DC supply?
Prodata Weather Systems
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UK Davis Premier Dealer - All Davis stations, accessories and spares
Littleport, Ely, Cambs UK

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Re: Davis Battery backup
« Reply #7 on: March 28, 2011, 07:05:42 AM »
Yes AC to 6v small < 20-40 watt type chargers, apparently not a big call for them, not a shelf item with minimim order quanity.

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Re: Davis Battery backup
« Reply #8 on: March 28, 2011, 07:15:16 AM »
Ok so who makes a 4.5 volt output DC/DC switch mode convertor with a nominal 12-13.8 volt (or between 9-36V) input? 

Offline johnd

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Re: Davis Battery backup
« Reply #9 on: March 28, 2011, 08:42:32 AM »
Do you specifically need 4.5v, or would 5v do? Ones I've used are from Dimension, eg:

http://www.dimensionengineering.com/DE-SW050.htm

Incidentally, outlets catering for robot hobbyists and also radio-control models seem quite good places to look for low voltage parts, chargers etc.
Prodata Weather Systems
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UK Davis Premier Dealer - All Davis stations, accessories and spares
Littleport, Ely, Cambs UK

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Re: Davis Battery backup
« Reply #10 on: March 28, 2011, 05:19:01 PM »
Specifically a voltage less than the main station voltage and similar to or just less than equiv battery voltages so 4.5v is a round figure but somewhere in the range of 4-4.8 volts. This is assuming the backup voltage needs to be less than the main voltage.

Those Anyvolt variable outputs would work.

Offline weathermedic

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Re: Davis Battery backup
« Reply #11 on: April 20, 2011, 01:20:16 PM »
Don't know if this will work and for how long, but how about one of those computer back up (UPS) power supplies made for computers? They come in several different configurations.


Offline johnd

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Re: Davis Battery backup
« Reply #12 on: April 20, 2011, 01:43:12 PM »
Don't know if this will work and for how long, but how about one of those computer back up (UPS) power supplies made for computers?

Not really unfortunately. PC UPS's are generally made to back up AC mains to AC mains and so aren't very relevant to remote installations. If anyone knows of any that might back up say 12v DC (supply) to 5/6 or 12v DC (output) then that would be much more interesting. I'm sure that such units exist but they're likely to be specialist parts and hence expensive.

It's frustrating - the principle of a 12v UPS is simple and there are suggested circuits on the web, but where do you find one made commercially at a reasonable price? Maybe they do exist and I'm just not looking in the right place? (Yes I know that they wouldn't be too difficult to make for yourself and that's potentially fine for a hobbyist interested in electronics, looking to make a one-off unit eg as a prototype and with plenty of time to spare. But many of us won't share those circumstances)
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UK Davis Premier Dealer - All Davis stations, accessories and spares
Littleport, Ely, Cambs UK

Offline Bushman

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Re: Davis Battery backup
« Reply #14 on: April 20, 2011, 04:41:20 PM »
Here you go: http://www.mini-box.com/picoUPS-100-12V-DC-micro-UPS-system-battery-backup-system  And full plans here: http://e.molioner.dk/projects/ups/

That looks ok but did you notice the input voltage is 15-16 volts which is probably a bit high for a typical 12 volt system (12-13.8 volt). As the manual notes it will work as low as 9 volts but below 15 volts will not charge the battery.

Offline johnd

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Re: Davis Battery backup
« Reply #15 on: April 20, 2011, 05:07:16 PM »
Yes I've seen this minibox part previously too, but similarly discounted it because it seems it won't charge a battery unless the source voltage is well in excess of 12v. I think if you look around you'll find that there is a version of this part which is claimed to work at somewhat lower voltage (eg 14-16 rather than 15-18, or something like that) but it's still too high.

As it is, this minibox part just seems to be an alternative to a 12v solar/wind charge controller without any additional benefit, but maybe I haven't understood the spec well enough.

Now if Minibox made an equivalent part for charging 6v batteries from a 12v supply then that might be more interesting, at least in the context of powering 5-6v parts like the VP2 console/logger. Actually, if you look hard enough you can find solar charge controllers for 6v batteries - maybe these would work OK from a 12v DC source, or would that be damagingly high?

NB For any newcomers to this thread, the reason for looking for such a unit (or at least - maybe I should say - my reason) is to find of way of giving a VP2 console/logger (which is powered from 5-6v) plenty of reserve power at a remote site even when the main 12v supply might drop out because it's also being used to power other devices such as a 3G modem.
Prodata Weather Systems
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UK Davis Premier Dealer - All Davis stations, accessories and spares
Littleport, Ely, Cambs UK

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Re: Davis Battery backup
« Reply #16 on: April 20, 2011, 05:23:15 PM »
Have been doing some tests on this one and the ideal solution is not simple.

One problem I see with 12/5 volt DC convertors is the wide input voltage range which constantly outputs 5 volts and can/will run a 12 volt battery right down to below a re-charge voltage and in doing so maintains a constant 5 volts to the station and not kicking in the backup. Once most of the convertors stop due to low input voltage the battery is then generally stuffed.

A different situation is where mains power is available and the station can either be powered by a 5/6 volt plug pack which if there is a power failure kicks over the backup immediately. Then of course there is a similar issue with the backup power system.

An alternative with power is a 6volt VRLA and 6 volt plug pack type charger which in a power failure stops charging, battery continues to slowly run down to a point where the voltage is less than the backup supply and backup supply kicks in. In this case the 6 volt battery should remain at a voltage that initiates charge when power comes back.

Suppose it all depends on how much backup time is required v battery size but there is generally a practical limit depending on the circumstances.  

  

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Re: Davis Battery backup
« Reply #17 on: April 20, 2011, 05:28:38 PM »
Quote
Now if Minibox made an equivalent part for charging 6v batteries from a 12v supply then that might be more interesting, at least in the context of powering 5-6v parts like the VP2 console/logger. Actually, if you look hard enough you can find solar charge controllers for 6v batteries - maybe these would work OK from a 12v DC source, or would that be damagingly high?

Now that would be close a 12/6v UPS with 6v charging. The 12 volt supply could then be battery/charger, power pack, solar etc.

There just doesn't appear all that much in the way of off the shelf 6 volt solar regulators, panels are available but nothing like there is in 12 volt. 


Offline johnd

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Re: Davis Battery backup
« Reply #18 on: April 21, 2011, 02:35:53 AM »
As I say, I have come across at least one possible 6v charge controller (Canadian make IIRC) which isn't widely advertised but is apparently available to order as the 6v version of a much more common 12v part.

But, as I've said previously, the solution I've gone with is to have two separate user-configurable and low hysteresis voltage drop-out switches on a 12v supply, one of which drops out at around 11.8v (this one powers eg the data link) and the other around 11.4v which, in conjunction with a 5v DCDC converter, powers the console/logger.

Although the difference between 11.4v and 11.8v (TBH I forget the precise numbers I used without further checking but it's something like this) might seem small, it can correspond to 15-20% of the capacity of a lead-acid battery. And then given that the console/logger power draw is relatively small - 100mA for an IP logger - compared to other devices that may be attached like the 3G modem, this can give a considerable extension to the reserve operating time for the console/logger when used with a high capacity (eg 100Ah or more) battery.
« Last Edit: April 21, 2011, 03:51:46 AM by johnd »
Prodata Weather Systems
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Re: Davis Battery backup
« Reply #19 on: April 21, 2011, 03:56:37 AM »
Quote
But, as I've said previously, the solution I've gone with is to have two separate user-configurable and low hysteresis voltage drop-out switches on a 12v supply, one of which drops out at around 11.8v (this one powers eg the data link) and the other around 11.4v which, in conjunction with a 5v DCDC converter, powers the console/logger.

Did I miss it or have you got a link for these drop-out switches on a 12v supply, or are these Dimensionengineering?