Solar battery charger

[Clive]

Retired now and no more daily commute to keep the battery charged and sadly just scrapped a previously good battery because of this. The car's street parked so I can't get a mains charger onto it. My plan is therefore to put a solar trickle charger on the rear parcel shelf. The one I fancy is rated at 2.4 watts, 17.5 volts at full output and 21 volts open circuit/no load. My main question is whether, given the Jag's very sensitive electronics, it's safe to have a charger like this permanently connected? It's hard to see how a trickle charger giving no more than 0.2 amps could damage anything. And the battery, one would hope, would act as a massive capacitor absorbing any peaks. And I'll fuse the feed right at the battery in case of a short/trapped wired etc. But I'd still like a second opinion on this , please. Secondly, is there any easy way to route the wire from the battery in the boot to the charger on the rear parcel shelf?
[Car - 1992 XJ40 3.2 auto]
Many thanks
Clive

[J44EAG]

Hello Clive,

Small solar chargers fitted to machinery with a primary energy motive power source such as a lead acid battery, can do no harm at all.

People often fit them to cars, boats, caravans etc where regular use is limited but where a small but non the less constant small electrical drain on the vehicle system continues to take place. As an example, I sight my Fathers Rover 825 that operated on the road infrequently but when out of use persistently failed to start due to a discharged battery. We never fully itemised the source of discharge and so fitted a small mains powered switch mode battery charger. That cured the problem. Father just used to pull out a mains plug fitted behind the radiator grill when he wished to use the car and would plug it back in at the end of the use period.

A battery is an unintelligent energy storage unit. It has no brain or feelings. As such it does not know or care where a recharge source comes from. It is unable to distinguish where a recharge source comes from ie whether the charge source comes from an engine powered alternator, a mains powered battery charger, a wind generator or a solar cell powered by the sun.

Solar cells are very popular in the small boat world where for instance a sailing yacht may be moored in the middle a river and no source of mains power is available to feed a mains charger. The vessel can be ignored for months on end, but when the owner returns to go sailing, he can step aboard, turn the key and his engine starts.

Typically, the solar cell is mounted in a location where the sun can shine on it and produce an electrical energy source. In addition, mounted out of the sun to avoid being fried by the sun on a hot day, or damaged by flying feet, is a small regulator box which is designed to regulate charge voltage to a pre-set level of around 14.4 VOLTS. That unit detects voltage produced by the solar cell and also detects the actual voltage of the battery it is linked to. As such it is known to be "intelligent" and therefore looks after a batteries best interests.

That regulator may also be fitted with "intelligent" charge regulation gear which automatically regulates battery charging to prevent under or over charging of the battery, to prevent a boil up situation developing and to bring and keep a battery at optimum voltage charge level and maximum capacity. Many have a programmable facility which takes into account usage patterns.

Solar units are very useful in maintain reliability and will not damage your internal vehicle electronic systems for the reason given in paragraph 3 of this posting.

Power leads from the unit to your battery should be kept as short as possible to avoid voltage drop within the charging circuit. Appropriate fuse protection should be fitted to the system, the rating of which will be advised by the charger manufacturer. Ring terminals should be fitted to the ends of charger leads where they fit to battery clamps. Crocodile clip ends are not recommended due to their ease of falling off and the possibility of inadvertent contact with each other in the event of one or other croc clips falling off.

The route of cables from the battery charger will no doubt be dictated by obstructions such as seats, trim and body shell structure. Cables should be well protected in vulnerable positions or where the pass through bulkheads. Grommets should be fitted to avoid any chance of them rubbing and wearing through on sharp metal holes. All cables should be fitting to avoid movement in service. Gaffa tape will be useful in locations where painted metal is available and the tape stuck to it with the cables sandwiched between the two.

I think you may find that the 2.5 WATTS you quote, is in fact more likely to be 2.5 AMPS. A WATT is a unit by which for instance a light bulb consumption is measured, ie a headlamp bulb is usually a 55W unit and a brake lamp bulb 21W. Hence the difference in illumination output(LUX)

AMPS are a measure of current flow the rate of current flow through a circuit. 2.5 amps would seem an appropriate figure for the unit you are thinking of installing. It is a baby trickle charger rather than a full blown lorry jump start unit!!

Let us compare output potential between various charging units available. Typical figures below.

Jaguar alternator, 80-100amps.

Typical home garage mains charger, 10amps max.

Wind driven generator found on medium sized cruising yacht, 30amps. Mind your head on the propeller blades!

Small solar cell for single battery top up maintenance charging, 2-5amps.

You can`t go far wrong installing one of these small units. By so doing you have everything to gain and nothing to loose. Very much install and forget. Just slightly increase your frequency of battery electrolyte level checks. Once every three months should be more than sufficient.

Mike

[ptjs1]

Mike,

If it's one of the £10-£15 solar chargers, it's very likely to be 2.5 Watt, not 2.5 Amp. Most of these lo-cost solar chargers have very low current flow and, even that is dependent on having decent sunshine!

Clive,

In my opinion, the current flow on that charger will do no harm whatsoever to your battery. As a source of very low current top-up, then it may well help you. And if you have no mains current access to the car, then you've little choice. However, don't expect great things from it. I did use one for a while and was disappointed that the current flow in anything less than good sunshine was very low indeed. Make sure that the model you've chosen also has circuitry which prevents back-flow discharge of current from the battery to the solar unit (I think all of them will have that).

With some judicious fiddling, you should be able to route the wire at the back right corner of the parcel shelf and down into the boot area.

Good luck

Paul

[J44EAG]

Load ie consumption is WATTS. There is no load as in a motor or headlamp etc

A charger does not consume, it provides in AMPS to a maximum amperage that the design of the unit will provide.

Current output from a source of energy is measured in AMPS.

Battery charger specification on the box will detail what the charger in the box is capable of delivering in AMPS. It cannot possibly be in WATTS.

V=W x A

Chargers of this nature will be fitted with a diode pack which is there to stop a reverse current flow.

QED. Where is my beer, I need it. Someone just covered me in bull dust.

May I suggest Ohms Law is suitable reading material for your holiday, Paul. Available from all good technical book stores and airport departure lounges.

Mike

PS. My daughter says you are "cool"!

[ptjs1]

Mike,

Tut tut, Volts = Watts DIVIDED by Amps

If you type "12V Solar Battery Chargers" into Google, you'll see that most small solar chargers are described by the Load Draw not the Current Output that they provide. (My "A" in Physics does stand me in reasonably good stead!)

Hence a lot of small chargers will be described as 2.5W or 4W with the small output current figure being hidden in the small print. I suspect that this because the figures look better that way and will fool people. After all a 2.5W charger looks better than a .15 Amp charger (which is what it will provide at approx 15 volts). This is particularly important when people are used to seeing mains chargers providing 10 amps.

Clive, I apologise, we're at the risk of hijacking your thread so will desist from here. The point is this very small output current is fine if you're not losing anything. But the current draw from alarm systems, radio memories, dodgy interior lights etc may be more than a small solar charger can replace, particularly if it's not in constant bright light.

Good luck with whatever you choose.

Paul

Mike, your daughter is clearly a great judge of character!

[J44EAG]

Just checking, Paul....Well spotted and a beer on me.

If that is indeed the case, standards maybe somewhat different across two industries.

The important thing then is to note that this type of marketing strategy exists and ensure that an adequate output is available from any solar unit.

A unit with a couple of amps output would be adequate for most vehicle applications. I have installed countless numbers on boats with even two battery set ups but I have always selected on amperage and not on wattage. I have never come across units sold in wattage classification. Interesting.

I concede Pauls point that a low power solar unit might struggle on a car which operates some services when "switched off".

Mike

[Clive]

Thanks for all your helpful advice. For the record, the output of the chager is 2.4 W (137mA @ 21V.) It's Maplin item N59FU on offer at present at £12.99, usually £24.99. So I bought two, one for each of my cars. I was actually more worried about the car's very sensitive electronics, not the battery. But it sounds as if trickle chargers are safe anyway.
Many thanks again,
Clive.

[Clive]

Sorry - let's get this right - 2.4 W or 137mA @ 17.5V full output. (21V is the "open circuit voltage.")
Phew!
Clive.