solar panel charge 24volt batterie

Solar Panels are constant-current devices - you can connect a 24 volt panel to a 12 volt battery and it will still charge at its rated current.

If your regulator can cope with the higher input voltage, you can connect a 24 volt panel to it.

If you have an MPPT Regulator, you will get double the current into a 12 volt battery.

no worries zacc:

get a DPDT switch and wire like this:




first batt + -----------\_______second batt +
first batt - -----------

first batt - -----------\_______second batt -
24V - -----------

The 24V + is the same as the first batt +.
In the shown position, you've got the two batts in parallel 12V configuration, throw the switch and you've got them in 24V series config.

Good luck and best regards, mr.batteryvalue

"is there any way i could leave them as paralel and charge them at 12volts but some how wire the out put to produce 24volt?"

. . . see reply 1 above.

thanks , with the DPDT switch can you get them so they will switch on from a 12 volt signal like a relay? sorry about all the questions.

zacc,

back to front: you want to replace the 'manual' switch with an 'automated' relay :)

The 'signal' can be a generated voltage which powers the DPDT relay on/off.

During the on-time the first battery receives charge from the 12V panel, and during the off-time the second battery receives charge from the 12V panel.

The batteries can be permanently wired in series to give you a permanent 24V.

The on/off ratio (duty cycle) determines how the charge from the panel is split between the two batts. If the duty cycle is 50%, each battery receives an equal charge from the panel (alternating in time which doesn't matter)

There are a number of ways of generating the required 'signal'.
A blinker relay acting as the 'signal' generator, is perhaps the easiest for you to use? Try to achieve a duty cycle of as close to 50% as possible or your two batteries will be charged at different rates.

I hope this explains the framework sufficiently. If you didn't quite follow all this, don't worry and seek the help of a professional installer with some electronics background (or an 'enthusiast' if you strike it lucky :)

Good luck and best regards, mr.batteryvalue

....and yes, if you want to operate the 'double pole double throw' switch remotely (or automatically by a signal), or via another relay, you can replace the DPDT switch with a DPDT relay :)

@ Allan B,

no fear Allan, as long as the relay contacts are rated higher than the maximum switched voltage and currents you can use this configuration.
The 'dodginess' you're addressing probably stems from your past experiences with 'make before break' situations, but this one is 'break before make', and the 'break' is shared between two sets of contacts in series.

Just consider this: during the '12V stage' the contacts in this circuit aren't susceptible to arcing because there is basically no voltage supporting an arc.
The only time there is potential for a brief mini arc forming, is when the relay reverts back from the 24V to the 12V configuration.
At this moment in time, the 24V load current gets broken by two sets of contacts in series, effectively sharing the burden of breaking between the two sets.

But for this brief mini arc to form, there needs to be a load current.
If this load current is within specs of the relay's contact rating, no danger.

Of course, like you mentioned Allan, on any battery operated circuit, fuses are mandatory. Choose a fuse rating of 50% of the contact rating - no paranoia no sweat, just good practice.
E.g. maximum load current encountered: 5A
Choose a relay with a DC contact rating of 10A or an AC contact rating of 20A or higher. The voltage rating should be 24V or higher, you're best served with a 240VAC rated one.

Put fast blowing 6A fuses in both batteries' positive legs and you're all set.

This circuit will sustain 12V/12A loads, or 24V/6A loads, and the switchover can be made 'on the fly' i.e. without disconnecting the load first (if that's even practical).

If the load gets disconnected from the batteries before switching from 24V back to 12V, the whole issue of arcing becomes moot.

Just repeating Allan's advice, the first thing to be done, put 'inline' style fuses in the positive battery legs.

Good luck and best regards, mr.batteryvalue

Mr Batteryman, No, I did not base my considerations on relays with make-before-break contacts. Quite obviously such a situation would result in an immediate short-circuit at switching and I would therefore not be expressing concern about a switching arc.

Having two contacts in series may lessen the likelihood of flash-over but it certainly does not eliminate it.

I said that your circuit was "dodgy and not good practice". To be more specific I will now say that in electrical engineering you NEVER design a circuit with the supply on opposing sides of a change-over contact. Sure, you may get away with it but there is potential for disaster and I have witnessed such on more than one occasion.

Incidentally, your suggestion of being "best served with a 240VAC rated relay" is also not valid. Relay and switch design for AC loads takes into account the "zero-crossing" characteristic when opening contacts and typically maintains a smaller open-contact gap so as to minimise the arc length until the arrival of the zero-crossing point when the arc will extinguish. These smaller contact gaps are not appropriate to the same DC loads. A relay or switch used on a DC circuit should be selected on the basis of the manufacturer's rating in respect to DC. The AC rating has no bearing on its capability to handle DC loads.

@ Allan,

any 240VAC rated relay can stand some DC voltage, I'm sure you know this.
Have a look at this inexpensive relay, it's rated 20A @ 220VAC/30VDC - so 12VDC or 24VDC can be accommodated easily.
http://www.jaycar.com.au/productView.asp?ID=SY4042&keywords=relay&form=KEYWORD

As to your note 'supply on opposing sides of change-over contact':
I understand the set of rules which govern the world you're coming from.

But if you forget the rules for a moment, and care to look at the circuit more closely, you'll find that these rules are irrelevant for this application.

It's because on change-over from 12V to 24V, there is almost nil voltage across the current carrying contacts hence an arc cannot form on contact opening. Thus there is no arc to expand all the way over to the opposing side (which seems to be the root of your fears).

Secondly, in case someone forcefully damages the relay mechanically and a short circuit forms between the opposing sides of the changeover contacts, the battery's inline fuse blows - nothing much of a 'disaster'.

The only time there is potential for an arc forming is, when the relay reverts back to the 12V configuration.
But firstly, a 30VDC rated relay has been designed to accommodate this, and secondly, you've got two sets of contacts in series, opening simultaneously - effectively reducing the arc supporting voltage to even less than 24V - well below the manufacturer's spec'd 30VDC.

If zacc doesn't feel comfortable following my 'what I'd be doing' advice, and I'm sure he/she knows there are certain dangers involved when handling 12V batteries, then it's never going to happen anyway.

Best regards, mr.batteryvalue (Mr Batteryman would have been to generic ;)

mr.batteryvalue, You further your argument by modifying your original expressions.

First you say "you're best served with a 240VAC rated one" but when challenged you nominate a Jaycar relay which has an alternate 30v DC rating. It is not simply a "240VAC rated relay" as you first nominated. Such relays have contact design capable of both ratings. That is not what you nominated.

Then you say that "on change-over from 12V to 24V, there is almost nil voltage across the current carrying contacts". There is in fact the battery charging current being interrupted at that time. Possibly not large but present nevertheless.

The fact remains that there are "good engineering practices" and there are "dodgy or taboo practices" and this is one of them. Employ them at your risk. You may get away with it or you may encounter a problem. Power supply on both sides of a change-over contact is taboo, as is contacts on either sides of a relay coil and fuses in the common return line etc etc. The list goes on, and a person trained, experienced and competent in the activity recognises and avoids them. There are better and established ways of resolving this application.

I have no wish to prolong this argument. I have pointed out the perils of the design and will make no further response on it.

Allan,

you may be a strict follower of 'good engineering practices' but I've to say, your understanding of the preconditions for arc formation is lacking depth.

In a nutshell: there needs to be some current through the set of contacts, i.e. charging current as you pointed out correctly.
Then, as the contact pressure reduces just before opening, there will be a rapid temperature increase of the contact area.
Finally, when the contacts open, fragments of molten contact material will be thrown into the still microscopic gap. These small particles inside a sufficiently high electric field, knock out electrons from the surrounding air molecules, which then carry the current trough the widening gap.
In order for this mini arc to be sustained and increased to a size to become visible, there needs to be a certain electron differential maintained across the gap.

Anyone ever done some welding knows this.

And that this only can work if the voltage is sufficiently high, is also a no brainer.

Now you might be tempted to think, the voltage across our relay contacts is equal to the battery voltage.
But that's wrong, as the voltage differential in the first few milliseconds of opening is only a fraction of the volt. differential charge/no charge, a few millivolts at best. Remember the batteries are at equal voltage in a parallel configuration, and when you break this configuration open (done by the relay contacts), there continues to be almost identical voltage on each pair of battery terminals, even if one battery continues to being charged and the other one not.
So where does the voltage sustaining the arc come from?

It's non existent "on change-over from 12V to 24V".

I hope to have shed enough light on this, for you.

As to this modifying-your-original-expression-thingy, I'm really not into this, sorry.
Fact is, any AC rated relay can be rated just as well for DC, it's called derating. The reason why I've said 'best served with a 240VAC (which has sufficient DC rating, remember?) is that I was under the impression that they are more widely available.
But looking at jaycar's web site, there's also a good choice to be had on DC rated ones.

Best regards, mr.batteryvalue

. . and we all know that in the real world, relay contacts never stick.

thanks guys for your valued replys ,i did not mean to start a heated discussion.
any way after all what i have read i decided to connect the batteries in series and get another panel so the batteries can have there own solar cells .

thanks

Wise choice Zacc.

Mike,

if you follow my advice given earlier in this tread, of limiting the max current through the relay contacts by fuses rated at smaller than 50% of this current, you won't see sticky relay contacts anytime soon.
In the unlikely case of sticky contacts under these precautions, the switch-over either won't happen, or the inline fuse blows - big deal.
BTW, sticky contacts don't arc, you can tick this one off ;)

If you're still using sleep over this issue, you could add a DC solid state relay breaking the circuit for a split second while the mechanical relay changes states.
That way, there is absolutely nil chance of arcing.
The solid state is energized by a one shot, triggering on the leading edge of the 12V/24V switch-over signal - wherever that's derived from.
I just hope that this little add-on won't raise even more suspicions in terms of 'good engineering practice'...but on second thought, this probably won't happen because DC solid state relays aren't exactly main stream.
They're a bit dearer, around 30 bucks or so, with a beefy contact rating of 40A @ 100VDC, yeah DC!

Best regards, mr.batteryvalue

good zacc, you've chosen the right path for you.
And don't worry about causing some debate, that's healthy engineering practice lol.

One final note: if the batteries wired in series, are AGM or gel, I'd use a charge balancer between them. Or, you can use two separate 12V charging circuits, one per battery (keep the panels separate as well, one per 12V charger). Not applying this little trick will shorten the life of your batteries, as the weaker battery will be susceptible to overcharging, and the other one to undercharging.

Good luck and best regards, mr.batteryvalue