Charge control of 12 volt solar panels.

[ericmark]

Charge control of 12 volt solar panels.
To use a solar panel as a stand alone unit is not a problem. The controller is basically a heat sink which converts any energy not required into heat and keeps the voltage within parameters.
However when used with other charging units this seems to be a problem. As the battery voltage is monitored to select charge stage.
Traction battery chargers work on a three stage system at first max amps is pumped into the battery then once the battery reaches approx 15 volt it triggers next stage which is a constant voltage charge current is now the trigger and once it reduces to a pre-set level the voltage is reduced to float charge at 13.6 volts.
See diagram

If one either uses the battery or charges the battery during the cycle it will change the point at which the stages change.
Using power is likely to delay the point of change but charging is likely to advance the stage so adding a solar panel charger would need to be in some way integrated into the system.
If the panel output was below 15 volt then during the main part of the charge cycle it would only produce heat and not help at all. On the other hand if producing 15 volt once the float charge part of cycle has been reached it would over charge the battery.
Using a small output panel with no control may be an option relying on the fact that more than it’s output is being used daily buy water pumps, lights, etc. And on the odd time it overcharges then the equalising charge is likely to do more good than harm with batteries where the water level can be topped up.
Assuming some light on the main charger to indicate stage of charge I would think one can produce a charge limiting device with a variable output.
But with a switch mode battery charger there is no need for the smoothing capacitors etc used in normal switch mode supplies and it can use far better control by monitoring the voltage between charge pluses.
So I am unsure of what effect the charge from a solar panel will have on the main charging unit.
I have read Design Trade-offs for Switch-Mode Battery Chargers with interest and this is what alerted me as to likely problems. I am waiting for my son to send details of charger used I know it is switch mode 5 stage and 50 amp but no more details at this moment.
I am sure other people must have faced these problems and maybe there is already some answers?

 

[electronicsuk]

I have never come across (nor can I find) a dual input solar/mains charge regulator. I'm not really into PV so perhaps I'm searching for the wrong thing, as it does seem odd that such a product doesn't exist.

I guess a relatively easy solution would be to look at the problem from a different perspective. Rather than trying to have both chargers connected at the same time, which, as you have found, causes problems with state-of-charge detection, why not have the batteries charged from the PV array only? You could then build a circuit that will switch the load over to a mains supply when the battery voltage falls below a set level. When battery voltage rises again to a suitable level, the load is switched back over to battery power.

 

[ericmark]

I am glad someone can see the problem. Original idea was to get a 2.5KW pure sine wave inverter and to only use shore power to charge batteries and since the charger is class II we would not need an earth but the more we look into things the harder it seems to get.

My son has hired a boat before buying his own and he found every morning the batteries were flat. So either he was using too much or they were not charging fully during the day. The latter seems most likely.

If the 50 amp 5 step charger takes around 9 hours to charge the batteries from flat a float charger in 6 hours even with 100 amp output has no chance as very soon after starting the engine the regulator will be cutting down the charge rate.

Even considered change over switches and when engine is running to run inverter off engine battery and use battery charger off inverter to charge domestic batteries.

But better option is second alternator with higher charge voltage than engine battery alternator.

As far as I am aware solar panels are controlled the same way as wind chargers where all power goes through a diode to stop back feed and once the voltage reaches 13.6 then a transistor resistor pair starts to sink the excess to earth heating the large heat sink in the process.

Many years ago I made a load of these for caravans as a battery charger of the shelf is far cheaper than just the transformer to make a regulated power supply. But put a 2N3055 transistor in the line and a 8 amp battery charger has a problem charging at 1.5 amp due to the 0.6 volt drop across the transistor so I reversed the circuit turning on the transistor when 13.4 volts was reached instead of turning it off and putting a head lamp bulb between transistor and earth the bulb also gave an indication of battery condition brighter the bulb more charged the battery was.

I used a very small 7805 voltage regulator to a power op-amp and to other input of op-amp to potential divider so by putting taps on the potential divider it would be very easy to set it to step from 15 volt to 13.6 volt in fact some of the propriety chips come with built in fold back. Which I may be able to use. But solar panels don't give a constant output but it varies between when sun is out or when covered so it would still tend to mess up both my home made regulator and the switch mode unit every time the sun comes out and goes back behind the clouds.

And at £400 for a set of batteries one can't afford for them to be left in a discharged state. I will still look at solar power but instead of the 10 amp first considered it may require limiting to around 1 amp i.e. just enough to stop effect of self discharge of the batteries when away from the boat.

 

[Chris.J]

Thesedays it is possible to use a series regulator between the panel & the battery instead of a shunt type across the battery. Try the Sunsei CC-10000 from CPC, pt No. BT 04166. This will probably work in parallel to another charging method, only way to be sure is to try it?

 

[ericmark]

These days it is possible to use a series regulator between the panel & the battery instead of a shunt type across the battery. Try the Sunsei CC-10000 from CPC, pt No. BT 04166. This will probably work in parallel to another charging method, only way to be sure is to try it?

Thank you Chris this has been a steep learning curve. And so much interacts with each other. With new information I have found about alternators I think the solar panels will not be required. I have now found an alternative to the blocking diode which boosts the charge rate to the main battery bank so allowing them to fully recharge within the estimated 6 hours that the engine will run in the day.
It seems Sterling Power Products Make a bolt on unit which will charge two batteries at different rates and this has completely changed my thinking. I only found out about their existence this morning so must mull over the possibilities. There is a cheaper option using a special regulator but as far as I can see that would also result in boost charging the engine start battery as well.

 

[kevindgas]

don't do PV myself but try getting in touch with Powertech Solar Havilland Road Ferndown Dorset. Eric is a wizz on PV and should be able to help you

 

[ericmark]

don't do PV myself but try getting in touch with Powertech Solar Havilland Road Ferndown Dorset. Eric is a wizz on PV and should be able to help you

Thanks but with Chris's help I found Select Solar web site and Powertech Solar seem to be more for mains units so I'll wait until I have talked with my son before giving anyone like Eric Hawkins a call.

I feel I have reached the stage where I can sit back now and talk to my son with enough back-ground to be able to make a good selection.

I have found a price on the inverter with Sterling but not as yet on the regulator that requires one to open the alternator to fit. And from what I read there must also be other people doing the same conversion. But I think at around the £275 mark the inverter is the answer so I will wait now until I have talked to my son.