Fitting dual supercaps to a Davis VP2

Hi. I recently repaired my Davis VP2 ISS. Supercap had failed and soldering a new one on has got it transmitting again. I’m also not sure if the backup battery is working correctly as every night I now get the ‘Low battery on station1’ message on the console. Battery contacts are clean and a new battery fitted BTW. I noticed when I was replacing the supercap that it is possible to fit 2. As it happens I ordered 2 supercaps from Prodata as I was sure I’d probably break one fitting to PCB. I didn’t and now have a spare.

Has anyone fitted dual supercaps and is there any reason why I shouldn’t? If there is a battery issue with mine then the extra power stored onboard would maybe make the battery surplus to requirements? Battery prob or not the ISS manages to transmit all night - and we are approaching the shortest day now. Low battery messages appears a couple of hours after sunset and goes once the sun rises. I’m toying with removing the battery and see what happens?

Cheers ~ John

If you wire them in parallel you will have twice the run time of a single one. The only concern I have is the charging capacity of the solar cell, you may end up with two half charged cells which will be basically the same power you have with a single one fully charged. I am not sure if the additional one will help much esp with the short days.

Aside from that issue of the limited charge availablity from the solar cell another problem with two in parallel is that will not be matched and there will probably be some inefficient charge swapping since they are forced to have the same terminal voltage. 1+1 will not equal 2 in this case. Also there is no redundancy in this circuit arrangement so you have probably doubled the chance of future failure :wink:

I have the second one fitted but not in paralell, it is as shown here:

[color=blue][i]ISS SIM Modification for Low Light Environments

For Weather stations which operate in low light environments an additional supercap may be added to your ISS SIM board to extend the on board CR17345 Lithium Ion battery life. The circuit board has the space and spare terminals specifically made to add a second supercap as shown below. The supercaps are charged by the solar panel during nominal light conditions and are discharged during normal use low light environments. The CR17345 Lithium Ion battery takes over and or supplements power when the supercap(s) run low or are completely drained to ensure continuous data transmission.


Davis Instruments does not recommend customers installing supercaps themselves. Any desoldering or soldering on your ISS SIM board will ABSOLUTELY void your one year Davis Instruments warranty. Modification is done at your own risk. Special care needs to be taken when working with the Supercap. Improper handling can lead to capacitor damage when forming the 90 degree bend in the leads, special care needs to be taken to assure no stress travels through the terminal lead rubber seal on the bottom of the supercap. The 90 degree bend can be done by holding the leads with narrow-tipped pliers near the rubber seal, and bending the leads against the pliers. When installing, special care needs to be taken to assure no mechanical stress travels into the part. Similarly, solder time needs to be minimized to keep thermal stresses from the Supercap.

The Supercap also needs to be affixed to keep mechanical shock from producing stresses between the part and the leads that are soldered to the board. This can be done with double-sided foam tape, silicone gel, or similar affixing.

It has been confirmed by Davis technical support that one or two super-capacitors can be installed as indicated above; also, several values can be installed. The night run-time is proportional to capacitance. The reason for Davis’s default selection was determined by need - vs - cost. The boards are designed to take 10F, 25F or 50F parts, up to two each. They can be mixed/matched in any combination.[/i][/color]

Thanks for the replies. Yes, I was looking to fix a second supercap to the board by the supplied points built into the pcb rather than in paralell - as per the Archer link. So according to Davis it is OK to do but they don’t recommend you diy solder the board :roll: Mine is a long way out of warrenty anyway. There again as it’s working well again I may leave alone.

On my board the two sets of supercap pads are in parallel. I just soldered my replacement into the unused ones, and snipped off the old cap so as not to stress the board with any desoldering.

I have read where at least one person replaced one super-cap with two. How is it working? I ask because my Davis Wireless Leaf & Soil Moisture/Temperature Station (6345) has been giving me problems. Last May I gave my VP2 and the 6345 a complete rebuild (minus the caps). The caps were in good shape but not the rest of the station.

Basically, the 6345 is the problem child today. It has been reporting to the console for some months that it has a low battery. The battery was replaced in May so I found it hard to believe it was bad. Today I got tired of the message and checked the battery. It was low (2.93vdc) so I replaced it. I guess it will take a while for the console to reset the message, no problem there. But my issue is the console displayed the low batter even in the day. Shouldn’t the solar panel take over during the day? I guess that I could always put a multi meter on the panel to see what voltage if any it is producing. I’d really hate replacing the whole station but I suspect the problem is the control board itself.

Oh yes one more thing. This same problem occurred last year and that was because of a broken solder connector on the battery holder. One I re-soldered it, all was well again.

Any thoughts?


I’m surprised you got low batt if it’s 2.93, that doesn’t strike me as low for a 3 volt battery. Last time I checked my ISS battery had to be in the low 2.8’s to trigger the alarm.

Yeah me too but I also emailed Davis to see what they think. The solar panel should eliminate the message during the day…

Are you sure that’s correct? I think the voltage alarm is based on the actual battery voltage, nothing related to the solar charging.

I was reading others postings on the super cap replacement and swore that I read that the reading would go away during the day. I see your point and since the solar panel is only charging the super-cap, that would imply the batter is on a seperate circuit. However, if the voltage is tested at the junction of the battery and super-cap then if the cap is bad, the battery picks up. If the battery is bad then the voltage would only drop at night.

IIRC the warning doesn’t reset until midnght.

You are correct, the message reset overnight and all appears well. At least until the battery dies again. I have considered adding a 2nd super cap to the unit and maybe an additional solar panel pointed in slightly a different angle. I am still iffy on using two caps! Since the unit is out of warrenty, maybe adding in a 2 AA battery holder and use rechargeable batteries! Naw, too much work, I’ll just buy another battery when this one fails!

Good talking with you,

I have also replaced the leaky super capacitor by 3 new one soldered in parallel and I do not need the lithium battery backup anymore.From the beginning Davis should have a super capacitor of at least 50uF.

Isn’t that rather a leap of logic though? You don’t have any location specified AFAICS, but your email address suggests it’s Australia. So in the middle of an Australian summer, there’s enough solar power gathered during the day to power the ISS through the night - I’m not sure that’s too much of a surprise because even the original single supercap should often do that.

But imagine being in the northern European or Canadian winter, where there’s not necessarily enough solar power to charge even one supercap for overnight use, let alone the three (or is it four?) that you have installed. (Having multiple supercaps can’t increase the total amount of charge that the solar panel generates - it can only increase the total charge stored if there’s sufficient charge generated to more than ‘fill’ one supercap.) That’s why the lithium battery is essential if the VP2 is going to be used reliably around the world.

And by using three supercaps in parallel, you’re tripled the chances of failure - is that really where you want to be?

Personally, I’m happy with the idea that the extra robustness of having the battery outweighs the relatively small price/chore of needing replacement every 18-36 months. If Davis had been able to source say a single 20F supercap to replace the 10F part then I wouldn’t argue that this might not have been marginally a better functional circuit design, but it would also have been a more expensive design decision and - just as in eg car design - ultimately every such decision has to be evaluated in detail for cost-benefit. The idea of having multiple supercaps in parallel certainly doesn’t appeal because of the potential loss in reliability.

Well my Wireless Leaf & Soil station is reporting that it’s battery is bad once again. This time the battery is 3.08 vdc. I pulled the station and checked the super cap. It registered 0vdc af ter a day of partly cloudy weather. I suspect the cap is bad but there is no oil leakage… The last battery only lasted 1 month. This is getting to be a pain.

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Anyone know where in New Zealand I can buy Davis super Capacitors