Voltage/charging confusion

[alibaba 78]

I thought I understood my charging system, but now I'm not so sure....

Setup - start battery and house battery. Connected to a VSR. so- when the engine is started, the VSR 'sees' the voltage drop on the start battery, tops that up, and when it reaches full charge, now switches its attention to the house battery and charges that. So far, so good, I think. The voltage regulator has been set to 14.7V to charge the batteried.

However, due to lack of use during Covid, my ancient engine driven compressor fridge crapped out, and I  have replaced it with an electric driven compressor plate. Because of the higher amp hours use, I have fitted a 60Watt solar panel, connected to the house battery. This seems to work well as well. When I come back from a weekend's use, the house battery is down a bit, but after a couple of days on the solar panel, it achieves full charge again.

But, it now occurs to me that I might have a problem. What is happening when the engine is running? The VSR brings the start battery up to full charge, but the solar panel is supplying about 0.5 amps to the house battery at a nominated 17.2Volts according to the specs.

Does this mean that the VSR will not recognise the house battery and therefore will not switch charging to the  house battery, because it 'sees' 17.2v? If so, I guess I need to put a switch into the solar panel circuit to turn off the input from the panel when I'm running the engine?

So now I is confused. What do you think?

[CarpeDiem 430]

17.2?  I hope this is a typo? 

The VSR connects the start battery to the house battery when the voltage at the start battery is above a given threshold. Usually around 13.2v this is sometimes user configurable, sometimes factory set and can vary depending on model. 

In practice, unless your start battery is substantially depleted, this means your vsr will connect the house to the start as soon as you start the engine. 

I would be very concerned if you are feeding 17.2v to your house battery. 

What problem are you having? 

[Island Time 1,235]

What is the solar controller?

Do NOT allow the batteries to get over 14.7v or so! It will kill them.

Get a PDS for the batteries, and set the charge sources to suit the batteries. No Vehicle I've ever seen has batteries designed for 17v...

[alibaba 78]

I've fitted the recommended solar controller - it says it is supplying 14.7 v.

[aardvarkash10 962]

The alternator output will swamp the solar panel output. 

Effectively, your solar is a trickle charger.  It's not going to impact the rest of the charge circuit in any meaningful way.

[K4309 264]

5 hours ago, alibaba said:

I thought I understood my charging system, but now I'm not so sure....

Setup - start battery and house battery. Connected to a VSR. so- when the engine is started, the VSR 'sees' the voltage drop on the start battery, tops that up, and when it reaches full charge, now switches its attention to the house battery and charges that. So far, so good, I think. The voltage regulator has been set to 14.7V to charge the batteried.

However, due to lack of use during Covid, my ancient engine driven compressor fridge crapped out, and I  have replaced it with an electric driven compressor plate. Because of the higher amp hours use, I have fitted a 60Watt solar panel, connected to the house battery. This seems to work well as well. When I come back from a weekend's use, the house battery is down a bit, but after a couple of days on the solar panel, it achieves full charge again.

But, it now occurs to me that I might have a problem. What is happening when the engine is running? The VSR brings the start battery up to full charge, but the solar panel is supplying about 0.5 amps to the house battery at a nominated 17.2Volts according to the specs.

Does this mean that the VSR will not recognise the house battery and therefore will not switch charging to the  house battery, because it 'sees' 17.2v? If so, I guess I need to put a switch into the solar panel circuit to turn off the input from the panel when I'm running the engine?

So now I is confused. What do you think?

the 17.2 v is the solar panel voltage, don't panic. Your solar controller will turn it into something suitable for your batteries, i.e. 14.2v.

Short story, the highest voltage source wins. To get the current into the batts, the voltage needs to be higher than the battery voltage. Hence why batts rest at about 12.6 volts, but you charge them at 14.2 (or 14.7 or what ever).

In theory, if your solar controller and your alternator have the same charge voltage, they will both charge the batts. In reality this won't happen.

I would expect your engine alternator to do all the charging. It will have much higher current. But for this to happen it needs a slightly higher voltage than the solar. You will find solar hardly ever actually give maximum output. Angle of the sun, clouds, deteriation of panel over time, etc. Normally angle and strength of sun. If you have an MPPT solar controller you can change the set point for the charge voltage. If you have a PWM controller, it is fixed. If you are worried about it, you can set the solar charge voltage 0.1 or 0.2 below the alternator voltage. HOWEVER, you need to check the required charge voltages stated for your particular batteries, and make sure you are giving them what they need.

Note that a 60w panel is in that grey space where it is not really big enough to charge house batts in use in a day, but is way bigger than what you need for a float charge. I'm mentioning this as it is possible to over charge your batteries with solar. Most MPPT solar controllers will give 2 hrs of 'bulk charge' before they drop over to a float charge. This cycle starts at the start of every day, so it is possible to over charge them. There are a bunch of ways around that, but the first point is to know about it / be aware of it. Fancy Victron charges assess the resting voltage at the start of the day and adjust the bulk charge time. But the more common EPever charges just give 2 hrs of full ergs every day. To get around that, I went for a smaller 20 watt panel which gives max of about 1 amp. Having just put in moonbeam $$$ new batteries, I'm monitoring everything with a battery monitor and a data logger on my EPever solar controller.

As a note, you can get the EPever data loggers locally for $80 or via Aliexpress for about $40. It is very cheap insurance for monitoring your system while not on the boat, so you can ensure you are not over charging your batts, and also see they are not going low charge and getting sulphated.

[K4309 264]

2 minutes ago, aardvarkash10 said:

The alternator output will swamp the solar panel output. 

Effectively, your solar is a trickle charger.  It's not going to impact the rest of the charge circuit in any meaningful way.

Only if it's voltage is higher. Higher current doesn't mean anything if it is pushing 'uphill' against higher voltage.

[aardvarkash10 962]

10 hours ago, K4309 said:

Only if it's voltage is higher. Higher current doesn't mean anything if it is pushing 'uphill' against higher voltage.

Yeah, I'm assuming the 17v reading is an error or typo.  The panel may produce 17v open circuit and ahead of the regulator.

[K4309 264]

2 hours ago, aardvarkash10 said:

Yeah, I'm assuming the 17v reading is an error or typo.  The panel may produce 17v open circuit and ahead of the regulator.

As I said above, the 17v is the solar panel voltage, the solar controller will convert it to something appropriate, like 14.2v.

My point is that if the solar controller is at 14.7 and the alternator is at 14.4, then the alternator wont be able to charge the battery, regardless of how much current it has, as the higher voltage wins. In reality I wouldn't expect a 60W solar panel to be able to consistently produce the rated power, but the comment was made to answer the OP's question, and so he can check his system set up.

[aardvarkash10 962]

35 minutes ago, K4309 said:

As I said above, the 17v is the solar panel voltage, the solar controller will convert it to something appropriate, like 14.2v.

My point is that if the solar controller is at 14.7 and the alternator is at 14.4, then the alternator wont be able to charge the battery, regardless of how much current it has, as the higher voltage wins. In reality I wouldn't expect a 60W solar panel to be able to consistently produce the rated power, but the comment was made to answer the OP's question, and so he can check his system set up.

Agree, if the battery is fully charged. However, a battery at a low charge state is such a large current sink by comparison to the PV, it will pull the voltage down and the alternator will supply the bulk of any charging current as you have already pointed out.

Pretty sure we actually both agree, but it is a situation where short answers in a forum will always exclude relevant details.