lustyd
Well-known member
Makes sense. Our "generator" is the alternator so slightly different but similar goals. I've yet to determine if I want a B2B but will probably wait until we have lithium batteries
VSR rewiring
- Thread starter lustyd
- Start date
PaulRainbow
Well-known member
If you can set the VSR to single sense, it works for now, no expense. I don't think a B2B adds and value at ther moment, different story if you go to Lithium.
geem
Well-known member
You wouldn't need a VSR. The B2B can be left on permanently, feeding the engine battery a float charge from the domestic bank. Mine runs at about 5W for two 100Ah start batteries in series. It's kinder on the start battery and guarantees its availability to start. In theory, it's should provide a longer life to the engine battery but I don't know how you would test or prove that.
PaulRainbow
Well-known member
I realise you don't need the VSR and the B2B. But, the OP already has a VSR that can be set to single sense, so it only closes when the engine is running. The engine battery will then sit stand alone unless the shore power is on, when it gets a trickle charge.
If he's planning Lithium in the near future, a B2B wouldn't be a bad buy now, or wait until final plans for Lithium
lustyd
Well-known member
Thanks all, sounds like my thinking isn't too daft.
Indeed, and will remain standalone if the battery switch is off even while charging. At the moment the VSR bridges the switch (on purpose, to allow the charger to charge both banks) which causes the problem.
PaulRainbow
Well-known member
Nor sure what you mean by " bridges the switch" ? The VSR should be wired to the battery side of the engine isolator and the battery side of the domestic batteries, so they all charge whether the switches are on or off.
How do you plan to make the VSR single sensing ?
lustyd
Well-known member
No, the BEP manual specifically shows in single sensing mode that the alternator sense wire is on the alternator side of the switch, not the battery side. In this configuration it will allow the device to sense alternator current and close when the engine is running, but importantly it won't close and can't close if the engine battery is isolated - or rather it will close but the battery is still isolated. Much better behaviour in a modern system in my opinion, otherwise the MPPT or other charging source will keep it closed almost permanently and effectively I'd have three parallel batteries in one bank, but with one different battery connected by thinner cables via the VSR. With this configuration, I can also have two separate charge profiles which will actually charge and maintain both banks properly while the engine battery is fully isolated.
lustyd
Well-known member
BEP website is down again so can't find the link. Regardless, it makes no difference to the operation of the VSR which side of a switch it lives on, it's just sensing higher voltage and connecting the two wires.
LittleSister
Well-known member
I am confused. If the VSR is closing (connecting the two battery banks) because it is seeing 14v from the domestic side of the circuit, what is the engine battery discharging to?
Lustyd worries about the engine battery potentially discharging, but has he actually experienced this happening?
Does the MPPT output not get pulled down towards the voltage level of the domestic batteries when they are partially discharged and there is only a potential 0.5A available from the MPPT?
Or is the output of the MPPT controller pulsed (rapidly intermittent) to maintain its voltage when the current demand from the depleted domestic bank is significantly greater than the MPPT can supply, so that the VSR sees 14v that is only there a minority of the time, but so frequently that the VSR hasn't opened (disconnected the banks) before the next 14v pulse comes round? (And the engine battery is discharging to the domestic battery/load in the periods between pulses.)
Lustyd worries about the engine battery potentially discharging, but has he actually experienced this happening?
Does the MPPT output not get pulled down towards the voltage level of the domestic batteries when they are partially discharged and there is only a potential 0.5A available from the MPPT?
Or is the output of the MPPT controller pulsed (rapidly intermittent) to maintain its voltage when the current demand from the depleted domestic bank is significantly greater than the MPPT can supply, so that the VSR sees 14v that is only there a minority of the time, but so frequently that the VSR hasn't opened (disconnected the banks) before the next 14v pulse comes round? (And the engine battery is discharging to the domestic battery/load in the periods between pulses.)
PaulRainbow
Well-known member
It does make a difference.
It should be connected between the battery sides of the switches so that it charges both banks regardless of switch positions.
If you have connected the engine side of the VSR to the load side of the switch (" bridges the switch") then when the VSR closes it will power the engine panel etc.
The only BEP schematics i have show that the VSR connections are both to the battery side. There is a wire that can be cut and routed to the ignition positive, which then prevents the VSR from operating unless the engine is running. Seems pointless, might as well use a headlamp relay.
lustyd
Well-known member
Various house loads. Chargers only output so many amps, and if the load is greater than this there is a net discharge, very much like a leaky bucket.
Yes, I have
No, it compensates and tries to maintain 14.4 (or whatevery the charge profile dictates).
lustyd
Well-known member
But not if that's not the behaviour you want, kinda the point of the thread. If there's an MPPT and charger on the house bank then the VSR will almost always be closed, as I said. This effectively gives you a single badly balanced bank.
Yes I guess it will - this is the kind of thing I started the thread for, stuff I hadn't thought of! Not a huge load, and I guess there's a risk of killing the VSR with the starter if I forget. My engine panel has an on/off switch of its own and is always off unless we're starting the engine. The alternative is forgetting to switch the VSR back on if I set it up with an off switch.
The one I had earlier showed it can be configured either side of the switch, and it seems clear that it will still work since it's just sensing voltage and closing the relay between the wires.
PaulRainbow
Well-known member
I'd be interested in seeing that schematic if you can find it please.
lustyd
Well-known member
Yes been trying but their website always seems to be broken. I got lucky this morning and it loaded but didn't save it
LittleSister
Well-known member
But how does the MPPT maintain 14 (or whatever volts) when the domestic bank is at a lower voltage because it is depleted, or the load on it is greater than the 0.5A available from the PV/MPPT? Is the MPPT output pulsed?
lustyd
Well-known member
The MPPT has an input voltage at least 5V above the battery so it has room to raise it if necessary. The battery voltage has to be measured at rest to get the various quoted voltages, in reality if you look at your system while anything is charging the voltage will be higher even if the battery is depleted. You can literally watch this happening if you have a voltmeter or decent MPPT with an app/display
sailoppopotamus
Active member
Sorry, but I don't understand how this could happen, can you elaborate a bit? Say we have a 100W panel. The MPPT keeps the battery at 14.6V, so we have a max charging current of 100W/14.6V = 6.85 A. If we draw more than that with the domestic loads, the battery is no longer charging so will drop to below its 100% SOC resting voltage of 12.7V and the relay opens. I may have misunderstood, but this is what this article on ACRs seems to be saying (towards the end of the page).
pmagowan
Well-known member
I too remain confused as I also thought that the battery would pull the sensed voltage down if discharged or discharging faster than being topped up
lustyd
Well-known member
The voltage doesn't drop though, unless you stop the charging. While charging the MPPT is designed to maintain a high voltage in order to push the charge into the battery, it's how charging works. You literally set your MPPT to run at 14+V during bulk charging. The voltage on the wire is the same through the circuit, you can't just measure the battery voltage without disconnecting it.
sailoppopotamus
Active member
Sorry if my previous post wasn't clear, but what I don't understand is how the MPPT could possibly maintain 14+V if I'm drawing more from the battery than its putting in, because then the battery is not "charging" in this scenario. My understanding is that the voltage (which is of course the same at the output of the MPPT controller as on the battery terminal) will drop when this happens. To put it another way: the current doesn't "charge" the battery with current subsequently "flowing out of it". If the MPPT controller is providing 7A and I draw 10A then the result is a battery that is discharging at 3A and it's voltage can't be more than 12.7V-12.8V. Or at least that is how I think it works.
EDIT: I'll test this on my boat tomorrow if I manage to get there before sunset.
