Alarm drain on V90 1997 battery

[peter burton]

My rarely used V90 has its battery go flat if not used for three weeks, I assume this to be about correct due to the power drain of the standard alarm system. No problems starting or running if jumped or charged by a charger. I have a small solar cell I am thinking of laying onto the dash and hardwire it to the battery to keep it topped up. Solar panel is about 1.5 watt and made to do this job. It will not charge via the cigarette lighter as this is only live with the ignition on. Where do you think is the best place to run the feed to the battery through the bulkhead. Hope to not remove too many panels/covers.

[TonyS9]

You should find a live battery feed going to the alarm, other wise try the light switch or something that works without the ignition on.

Your battery could also be on the way out. The first signs are lack of capacity or loss of charge while just sitting. Still 3 weeks isn't too bad. My 360 doesn't have an alarm, it does have a few items that run, but the newish battery lasts for months.

This can be especially a problem when the car is used for short journeys and maybe combined with poor charging voltage from a slightly corroded loom. Check your battery voltage (both probes on the battery terminals) at idle with the lights on, and compare with the alternator voltage. Any more that 0.3V is not good, but it will still function with 1V drop, just take longer to charge.

A solar panel isn't a bad idea to keep the battery good, but do check the volt drops.

[Laird Scooby]

If you're going down the solar panel route, a few things to remember. First is to disconnect the solar panel before starting the car as the current draw from the starter can effectively short circuit the solar panel, rendering it useless.

Also to keep a fully charged battery at that level, it needs to output 14V minimum. That means with a 1.5W panel it's about 100mA charging current. Fine if it's 24 hour operation but Nature dictates differently.
At the moment, at best we're getting about 8 hours of sunlight a day (strong enough to get the panel charging anyway) The panels have a fairly wide angle of incidence so south facing is going to be your optimum positioning for the panel.

Going back to the charge rate, 100mA will cover the parasitic load on the battery and replenish some of the losses overnight. However it's fair to assume at least 40mA drain minimum 24/7 with the car locked. With only 8 hours of sunlight in a 24 hour period and 100mA available during those 8 hours (assuming perfect operation from the panel and strong enough sunlight), that 100mA is effectively 8/24 x 100mA over a 24 hour period.

That equates to 33mA over 24 hours - less than the parasitic drain. Result? Flat battery but it will last slightly longer than without the panel.

To be certain of the effectiveness, you really need a 4.5W panel to get the average current over the 24 hour period above the parasitic load figure. This will also help on cloudy days, hazy days and so on that we get plenty of in East Anglia!

[TonyS9]

Originally Posted by Laird Scooby

I..First is to disconnect the solar panel before starting the car as the current draw from the starter can effectively short circuit the solar panel, rendering it useless.

Have you seen this before? I assumed its a naturally limited power source, and the output figure is the figure it can handle. Will shorting it produce more power?

I agree though 1.5W is a bit on the low side to be self sustaining.

[Laird Scooby]

Originally Posted by TonyS9

Have you seen this before? I assumed its a naturally limited power source, and the output figure is the figure it can handle. Will shorting it produce more power?

I agree though 1.5W is a bit on the low side to be self sustaining.

Yes, the one i bought carried a warning and i forgot one day. It is now buried in my shed but should really be buried in the bin!

When i was researching battery desulphators having come up with the idea and designed my own, i doscovered solar panels are very often misused, mainly because people don't understand the design specs on them.
While trying to find the MOS-FET and output choke i need to complete my design, i found a commercially available unit, almost identical in design to mine, ready built and ready to plug'n'play for less than i could buy the components for so i bought one of those instead.

However, what i found on solar cells to power off-grid installations was roughly along the lines of work out your maximum power consumption for one hour with everything in use.
Multiply it by 3 and then add 50% This is the minimum wattage the solar cell you need will have to be. That is my interpretation of it, many different formulae can be found online to arrive at a similar conclusion.

For simple battery float-charging, you can ignore the add 50% bit if you assume roughly twice waht the expected quiescent current of all alarms, ECU/radio memories etc will take, in other words expected is 50mA so assume it can take 100mA - hence the 4.5W minimum recommendation on the solar cell.

[peter burton]

Thanks I see it would not work so I will get a smart trickle/conditioning charger and use a fly lead connector. My mate uses Oxford ones to keep his motorbikes charged while stored. If I plug in once a week all should be good. I have only ever had cheap very basic battery chargers before. After 25 plus years I am going to sell my V90 and be volvoless. Road tax is too high now, 25mpg is a bit high so I will be downsizing to a VW Polo type/size cheap tax car. I will post it for sale on here soon.

[Laird Scooby]

Originally Posted by peter burton

After 25 plus years I am going to sell my V90 and be volvoless. Road tax is too high now, 25mpg is a bit high so I will be downsizing to a VW Polo type/size cheap tax car. I will post it for sale on here soon.

In that case, can i suggest a pre-2000 Honda Jazz or Civic? Anything pre-2000 will be cheaper on tax (usually, there are exceptions obviously) than the post-2000 version but more importantly, if the cat fails, you don't have to fit a Type Approved cat which often quadruples the price of the cat, if not more.