Questions for electrical gurus

[Lahara]

Currently, I have an 80 watt solar panel charging a 100 amp hour battery through a Steca Solsum 8.8X regulator. The regulator is the type that prevents over-discharge as well as over-charging, and incorporates a blocking diode to prevent backfeed from the battery. (This is electrically separate from the crank battery, which is charged by the engine alternator and kept topped up by its own smaller solar panel).

As a simple linear diagram, my setup looks like this:


80 watt solar panel ----> Regulator ----> 100 amp hour battery


I wish to add another battery to drive a new electrically-separate circuit. If I could, I would simply parallel the existing setup with another solar panel, regulator and battery but I can't find the room for another panel at this time. Another complication is that I have no room to accommodate the dimensions of another 100 amp hour battery. The largest battery I can find that will fit in the available space is an 80 amp hour model.

My thinking is to charge both batteries with the one panel but using a separate regulator for each battery to retain individual charging and discharging control. The proposed setup looks like this:


````````````/----> Regulator ----> 100 amp hour battery
Solar panel /
`````````` \
``````````` \----> Regulator ----> 80 amp hour battery


My questions are:

1. Though the batteries are electrically connected while being charged, do the blocking diodes in each regulator keep the batteries electrically-isolated when discharging through their respective regulators? To put this another way: will a light left on for three weeks on an unattended boat drain one battery or both?

2. What will be the effect on the amps being fed to each battery if (a) both are fully charged; (b) one is fully charged but the other isn't; and (c) neither is fully charged. I guess my question is how is the available charge split between the two regulators and how is it likely to vary in practice.

Both batteries are sealed calcium wet cells.

 

[andy_wilson]

Looks good and would appear to do what you wish.

Charge will go to the batteries in inverse proportion to the internal resistance, which with identical technolgies will be representative of the state of charge.

Therefore, if one is a bit discharged, and the other is nearly flat, the nearly flat will get a bigger proportion of available charge, and the charge rates will reduce relative to the state of charge such that they will each reach maximum charge together.

At least that's what would happen without regulators, and I can't think of a good reason why they regulators would interfere with that.

 

[Danny]

I don't think this will work because they are shunt regulators! These work by dumping excess current. If you put two in parallel they won't work effectively ( as seperate regulators) - although they would provide individual blocking diodes for each battery.

 

[markdj]

Use one regulator for the solar panel but connect in either a splitter diode or a Voltage Sensitive Relay. You can connect engine alternator to VSR as well or if you have 2 engine then get 2 VSRs. You will probably need an isolator so that the solar panel can be disconnected when the engine(s) are running.

 

[Lahara]

Thanks so far to Andy, Danny and Markdj for your comments.

Andy, I like your answer the best because it suggests the proposed layout would work.

Danny, I understand from what you have said that a regulator serving a fully charged battery would dump excess amps rather than those amps going to the other regulator and battery. Have I understood you correctly? This seems to contradict what Andy has said that the charge would go to the flatter battery and, interpolating from this, I had thought that no dumping would occur in either regulator until both batteries were fully charged because there wouldn't be anywhere remaining for the current to go to. Is this too simplistic a layman's view?

Markdj, I had investigated using a VSR but wanted to retain a regulator for each circuit for current discharge control. Running both circuits through the one regulator meant that, potentially, total discharge would come very close to the discharge amperage limit of the regulator.

Apologies if some of my statements sound a little dumb - last time I had to do any of this stuff was in my physics classes in 1966.

 

[William_H]

Now for a simple fix. Discard the regulator. You should find in the 80watt panel a diode used for blocking power back into the panel in the dark.
You need to duplicate this diode. (get another at least 5 amp silicon diode)
Connect the new diode to the panel itself and run another wire for the positive from the new diode to the new battery. If you put yet another diode in you can charge 3 batteries. You are using the blocking diode in the panel as a splitting diode at the same time which actually saves some power. of course it isolates each battery for the loads.
To explain the diode loses .8 volt. This doesn't matter so much because the panel produces 18 to 20 volts (or tries to) however in marginal light this voltage can fall so the diode drop can reduce the output under really marginal conditions. (theoretically anyway) So to have only one didoe in circuit between the panel and the battery is better than 2.

If you are charging your batteries with a 80watt panel about 4.5 amps then you are probably quite safe without a regulator with 180 AH. Even safer if you connect ther engine battery with another diode. However if you are concerned and have one regulator already then fit it to the smallest battery. you will find that the shunt aspect of the regulaor will pull the panel voltage down until it is no harm to the bigger battery direct connected. ie partially or completely regulating the charge to the other battery.
The whole system can be checked by looking with an amp meter at the voltage and current into each battery. and of course monitoring electrolyte level if you can. consider olewill

 

[Lahara]

Thanks Will, for another solution. However, if I have understood you correctly, it means that the two batteries would be connected together. What I was trying to accomplish was to have two batteries that would feed their respective load circuits in isolation of the other, and also to retain individual discharge control for each battery. (With the regulators in question, discharge feeds back through the regulator).

If I can re-cycle/adapt my original diagram as follows:


Solar panel 1 ----> Regulator ----> battery
``````````````````````|
```````````````````` Load


Solar panel 2 ----> Regulator ----> battery
``````````````````````|
```````````````````` Load


This shows the ideal solution for me, with each circuit being completely isolated from the other. But as I have said, I can't fit in solar panel 2 now (probably possible next year when I can put in a stainless arch over the stern end). I suppose my question is really how do I, or how can I, as a temporary solution, end up with the same degree of electrical separation and the same degree of charge/discharge control for each circuit shown above while using only solar panel 1?

 

[William_H]

Hello Stuart No using 2 diodes is in fact the same as a purpose bought diode isolator. However I am proposing using one of the diodes already supplied in the panel and adding another diode from the panel (here I mean from the actual solar cells+).
A diode will only conduct in one direction. You will have the anode of one diode going to the sollar cells+, the cathode to pos of one battery and the anode of the other diode also connected to the solar cells + the cathode being connected to the pos of the other battery. ( fit fuses at each pos terminal.)
Now current trying to flow from one bat + to the other will encounter 2 diodes one after the other both blocking the current path. There is total isolation of one battery to the other. (except they share the charging source)

Now if you can't get to the diode in the solar panel (the ones I have seen are mounted in junction box under the solar panel.) then don't despair just add 2 more diodes one to each battery. There will be one more (the original sollar panel diode) in series which will lose somme volltage but it won't matter much.
These diodes can go to 2 regulators if you wish (or just one) or none.

However it is possible that the regulators themselves have diodes which will do the same job. So just connect one panel to the inputs of both regulators. You can confirm total isolation by disconnecting the battery terminal of one battery and ensuring that everything connected to that battery is dead. (when normally it woulod be running) reconnect and do the same test with the other battery terminal ensuring that everything is dead again when normally it would be running. good luck PM me if you are still confused. olewill

 

[Lahara]

Hi Will, yes the regulators themselves have Schottky diodes to prevent backfeed. Even with my lay view of things electrical, I have suspected all along that these would be sufficient to keep the batteries electrically isolated as far as discharge is concerned and will use the test you suggested to confirm this is the case. To my minor embarassment however, I have to admit all of this discussion may not have been necessary had I gone back and re-read the manuals that came with the panel and regulator. I have just returned from checking the manuals on the boat where I discovered that the solar panel (Sharp NE-80EJE) itself has two diodes serving two positive and two negative terminals, of which I'm currently using one positive/negative pair. It doesn't actually say anywhere in the manual this second set is for a second battery but it would seem likely it is, so I will be checking this out next year when I dismount the present pushpit-mounted panel and move it on top of the arch. Thanks for your help.