Voltage sensitive relay unusal question (probably)

[halcyon]

. This pull down of the voltage will cause the VSR to release. The VSR may then operate again cycling continually until the service battery voltage has been lifted by the pulses of charge. This is not very healthy for the VSR relay contacts.
As said a hysteresis ie a difference of operating voltage to release voltage will help except in the case of a really flat big service battery. Even then the longer on time will get the voltage of the service battery up with less cycles. Another way to avert this problem is to introduce some resistance into the charge wire through the relay contact. This will limit the large current going into the flat service battery enabling the alternator to hold the voltage high enough to keep the VSR operated. As the service battery is charged the current will fall so voltage difference becomes minimal./QUOTE]


The actual setting of the pull in voltage of the VSR will determine just how much charge the engine battery gets before the service battery gets a charge. So a VSR set at 13v will pull in almost immediately alternator kicks in. A vsr set at 14v volts will not pull in till engine battery gets to 14v. This voltage will not appear until the engine battery is well on the way to fully charged. This depending on the alternator current capability. The alternator will be trying to provide 14 or more volts and the engine battery will be taking charge so it is still a bit variable just ho well charged the engine battery is before the VSR kicks in.
Not that this really matters as the alternator should be able to charge both batteries. So this idea of second battery only being charged when engine battery is charged is just a bit of salesman hype..

The reason for charging engine battery first goes back to the late 1970 when we were designing the first ones, back then engines were not as today and alternators 25-35 amp. On the relay engaging ( 13.6 volt as alternators regulated at 13.6 - 13.8 volt then ) with a low house battery, the voltage drops due to overload on alternator. To improve this situation we introduced a pulse start up, alternator charges up engine battery, engages relay, engine battery discharges to help supplement the alternator, relay opens and the alternator recharges engine battery. This continues until the current is low enough to maintain alternator voltage at a minimum ( 13.00 ) charge voltage. As the relay spec is compatible with relay rating, no damage to relay occurs.

Back to OP, you will not charge two batteries with a 5W panel and a VSR.

Brian

 

[tim_ber]

Thanks for additional info guys and advice to 'have a go'.

Yes, if I do manage to build a vsr ( big IF), then I will certainly need a larger solar panel, but they have reduced in price a lot recently.

cheers

 

[William_H]

Thanks for additional info guys and advice to 'have a go'.

Yes, if I do manage to build a vsr ( big IF), then I will certainly need a larger solar panel, but they have reduced in price a lot recently.

cheers

If your budget can run to it have a look at panels for domestic PV systems. They are very cheap here for the power ie 200watt but they are 40v panel so need an MPPT controller or use direct on a 24v battery system. You will need a regulator of some sort at that power. PM me with email address if you want a description and circuit of a simple VSR olewill

 

[jwilson]

Why can't you parallel them? They will charge just as quickly.

A VSR is only used when the batteries must be separate, as in start and domestic. And there is a common misconception that they somehow sense that one battery is charged, and switch to the other bank. They do not, they just sense that there is a charging voltage, and parallel the batteries.

Correct: the problem the OP may have is that his 5 watt solar panel - about 0.4 amp delivered in good conditions, may struggle to produce enough charge for a second lead-acid battery. VSRs work normally with car/boat alternators that can chuck out 50+ amps if faced with flat/tired batteries.