Newbie battery charger questions

[roadtripper]

The advantage is in the "off" season you have the chance to do a few experiments like this.

It's entirely possible that we find no long term issue, or at least one that needs addressing. But what it does give you is a good understanding or your system and what makes it tick and that's valuable in understanding how best to use it but also for noticing if it's behaviour changes.

 

[wallport]

Agree totally!

I have been drawing a wiring diagram and confirming a few things today. All tests done with heater fuse still removed.

It's confirmed that the 20A fuse in bank 2 is between the victron and the battery.

I'm about 95% certain that each fuse bank has a dedicated wire (I've looked behind the panel and the remaining 5% is whether a wire loops back out of sight). This would mean there are no unexplained battery wires.

Confirmed fridge is fuse 1, light dimmer is 2. However, fuse 3 (supposedly "splashback" is a mystery as I cant find anything working when it's plugged in.
Fuse 4 supposedly "kickplate" is actually the usb sockets and the battery status lights on the splashback.
Fuse 5 (bank 2 fuse 1) has the kitchen light as well as the poptop light on it.
Fuse 6 probably is everything to do with dometic hob (lighter, Gas pump) but I could only test the tap which worked.
Fuse 7 is the victron and fuse 8 is not connected.

Now, the battery status indicator showed 2 (red would be 1) with the battery voltage measured (both with a meter and bm2) at 12.56v. From a previous post I thought it was supposed to be upper yellow (3) until it dropped below 12v? I think maybe you made the point previously that this panel could be misleading as in theory light 2 means charge as soon as possible. It looks to me as though the bm2 reading will be a much better way to decide and that "recharge ASAP" may be at 12v or below??
Also means I need to permanently wire the bm2 and fuse it. I have the parts now - what fuse should I use? 2A is the smallest I have.

Maybe no bottle of malt (unless we bump into each other at a Campsite one day) but thanks again for all the advice. I suspect this thread will be of help to many people, not just me, for years to come.
Will post further results after giving it a week or so.

 

[roadtripper]

2amp fuse would be fine, the BM2 current draw is tiny.

I would certainly trust the BM2 more than the basic LED indicator, especially if I had checked the BM2 with another multimeter.

The important thing is try not to drop below 50% state of charge, which for your battery type is 12v.

The basic LED indicator isn't useless though, once you know what voltage level the LEDs on it light in your system you can use it as a basic check.

And thanks for the updates and graphs, it would have been easy for you to decide everything was fine and turn attention elsewhere, but now I think this thread is a good example of how a basic understanding of your leisure electrics isn't hard to come by and is hugely useful. Best payback is to get out, enjoy your van and post the great places you find back here to inspire others

 

[wallport]

Have now fitted the bm2 with an inline fuse. For anyone else doing this in the future I would just report that when I cut the positive cable of the bm2 I was surprised to find two separate insulated blue cables. This made me wonder and I came up with a theory that maybe one was acting as a Bluetooth antenna. However, decided to just strip both, wrap them together and join to fuse wire and all worked fine.
No idea why they have 2 wires.

 

[wallport]

Before doing the next test of battery drain, I needed to run my heater for 30 mins as it's required to do every 30 days. The battery should have been fully charged beforehand but I took it off charge to temporarily to replace the heater fuse and left it off hookup while the heater ran and got this result.

Now, I realise that measuring a battery solely by voltage must be an imprecise science. However, this made me wonder:
1. Is the above what should be expected as the 'hit' for 30 mins of heating?
2. Why does the voltage go back up when the heater goes off?
3. How much can I read into the voltage levels being reported whilst there is an appliance in use or is it more meaningful when nothing is drawing power?

Incidentally, I have noticed in the past that the light indication system sometimes does not show a green light when the hookup is connected and I think this is while the fridge is actively drawing power as a few mins later it does show green. A bit confusing and caused me to think the hookup was faulty at first.

 

[n10n]

Before doing the next test of battery drain, I needed to run my heater for 30 mins as it's required to do every 30 days. The battery should have been fully charged beforehand but I took it off charge to temporarily to replace the heater fuse and left it off hookup while the heater ran and got this result.
View attachment 226989
Now, I realise that measuring a battery solely by voltage must be an imprecise science. However, this made me wonder:
1. Is the above what should be expected as the 'hit' for 30 mins of heating?
2. Why does the voltage go back up when the heater goes off?
3. How much can I read into the voltage levels being reported whilst there is an appliance in use or is it more meaningful when nothing is drawing power?

Incidentally, I have noticed in the past that the light indication system sometimes does not show a green light when the hookup is connected and I think this is while the fridge is actively drawing power as a few mins later it does show green. A bit confusing and caused me to think the hookup was faulty at first.

1. What you see is normal and correct. You are measuring voltage, not state of charge. Voltage only gives an estimate of charge level when there is no load.

2. Both the battery and the load have internal resistance. When the load is connected (heater on), the voltage drops with respect to the ratio of the battery and load internal resistances. When the load is disconnected (heater off), no current will flow and hence the voltage jumps back up.

3. It is more meaningful when nothing is drawing power. However, knowing your loads you will, in time, learn how the voltage behaves and to visually know what's normal and what's not.

EDIT: for more comprehensive explanation of 2, read about voltage dividers. In that page, you can think an ideal battery providing voltage Vin. Z1 is your battery internal resistance, Z2 is the load resistance and Vout is what you measure by BM2.

 

[AeonMach]

Further to what @n10n says above, here‘s an article that explains voltage change with load and gives some tables:

AGM Battery State of Charge – workshoppist.com

 

[roadtripper]

The only really accurate way of measuring charge is the shunt way that the van does it or you use something like a Victron Smart Shunt - count the charge in and count the charge out.

However voltage gives a good estimate (on lead acid chemistry) if the battery is broadly at rest. It's really not as good for lithium chemistry as the discharge graph is much much flatter.

Another way of considering the voltage drop under load is to remember that a battery is a chemical reaction happening on the surface of the battery plates. With no drain on the battery the reaction is essentially dormant so the voltage is stable. If you start drawing energy out of the battery this means the chemical reaction is happening, which means the electrolyte by the plate is partially exhausted so the ability of the battery to provide energy drops. This is also where the fundamental ability of a battery to provide energy comes from - there is a fundamental limit to how fast the reaction can go for a given area of battery plate and once you hit that limit you'll not be able to draw any greater current from it.

 

[n10n]

This is also where the fundamental ability of a battery to provide energy comes from - there is a fundamental limit to how fast the reaction can go for a given area of battery plate and once you hit that limit you'll not be able to draw any greater current from it.

Ideal battery would have zero internal resistance and it would push out infinite amount of amps at any time. Batteries designed for starter usage have low internal resistance, and hence they can push out large currents eg 700A for cranking. Still, 700A is far cry from infinite and this limit comes from battery chemistry saturation as @roadtripper described.

Your heater will not hopefully demand anything near to 700A but the battery resistance / chemistry is still there at any time limiting the current - thats why the battery connected to a circuit forms a voltage divider with the load and you see a voltage drop.

 

[wallport]

OK so the results for an extended period with just the heater fuse removed are:

Compared to this previous graph with all the fuses removed:

This last graph is over a shorter period and shows higher voltage but it is noticible in the top graph that it is barely dropping at all day to day over the last few days. I am beginning to suspect that although there is a little evidence that heater does cause slight drain, I may end up with the conclusion that not much is wrong.
I will now charge up over a couple of days and repeat the test with the heater fuse back in (ie all fuses in place).

 

[wallport]

So, here's the graph for all fuses in place. This one is over nearly 8 days and the previous graph is over 6 days but again it's barely falling at all over the latter days.

It's definately lower with the heater connected but is it significant and does it suggest a problem or is it normal?

 

[wallport]

Any thoughts @roadtripper?

 

[roadtripper]

Sorry when I go away I generally go off grid digitally otherwise it feels too much like I'm still at work!

Comparing the two charts the heater clearly has an impact parasitic drain wise, but that's still a reasonable health voltage (roughly 80% charge vs. 70%) so whether it's a problem is really down to how you use the van.

If it's a daily drive (and I assume not with the long tests!) then no real issue. Even if it's a weekend van then no real issue. If you are storing the van for a few weeks then you might want to consider what's happening and think about either supporting the battery a bit while stored or have an easier way of isolate heater when you know you'll not be using the van for a while.

But now you have the monitoring I'd just keep an eye on it and see what happens for your normal usage. I'd suggest not letting it drop much further charge wise than that last test to keep the battery in a healthy range and also so you don't have to drive for an age to get it topped back up when you do want to use the van

 

[wallport]

Sorry when I go away I generally go off grid digitally otherwise it feels too much like I'm still at work!

Quite right!

If you are storing the van for a few weeks then you might want to consider what's happening and think about either supporting the battery a bit while stored or have an easier way of isolate heater when you know you'll not be using the van for a while.

There will be periods when the van won't be used for a few weeks and I think if I've taken one thing from this thread it's that we should put it on ehu regularly when parked up and always ensure it's fully charged before going on a trip with no hookup.
One question - do others feel the need to use ehu even when just parking up for a week?
One thing I want to test is whether we can manage 2 nights (or even 3) with no hookup with fridge and some heater usage starting with a 100 charged lb. If so, probably no need to change anything.

However, if this doesn't work out, I think we will look into roof solar. A quick investigation suggests we might be able to add this for hundreds rather than thousands. I'm unsure how much impact this will have though, particularly when the sun don't shine.

On the heater draining the battery, I wonder if the manufacturer (Autoterm) might be worth contacting to check its what they expect?
One specific question, any idea why the bm2 says 12.29v is 47%? Surely 50% should be 12.06v (from memory)


Thanks again for all your help with this.

 

[roadtripper]

Check what battery chemistry the BM2 is set to?

Solar is more effective with full sun, but it's surprising how much you can get on a bright but not sunny day. The advantage it has is if mounted on the vehicle it's always there. You've seen what a few watts out looks like, even a few watts in makes a difference when it's ticking over every day, especially for the maintenance.

My Caravelle is single battery and only has OE load, I still plug it in in the week most times now I've made up a trailer socket charger. It's too easy to change plans at a weekend, think you'll be out next weekend and suddenly you've been 3 weeks.

 

[wallport]

On the heater draining the battery, I wonder if the manufacturer (Autoterm) might be worth contacting to check its what they expect?

I dropped them an email and got a response saying that some battery drain is to be expected as microprocessors are running all the time. However, they are asking me for the drain in amps which I have no idea how to answer.

 

[roadtripper]

You'd have to do the direct measurement we talked about earlier to get that. Now you know what it is a simpler multimeter will do it, just remove the fuse and carefully probe across it with the multi meter on current setting.

You can get a rough (guess)timate from your testing if you can work out the difference in capacity drop between the test with the heater in and without. If you lose 1% more capacity on a 100aH battery over a day then the parasitic drain will be ( 0.01 [1%] x 100 [Total ah] ) / 24 [h] = 0.041 amps ( or 42 milliamps )

Word to the wise if you do test current with a conventional multimeter always put the mode and leads back to voltage straight after the test out of habit. Configured for current testing the probes on a multimeter are very nearly a dead short, so picking it up and using it for a voltage test when it's been left configured for current testing will get very spicy very quickly; the other way around is fine as voltage is very high resistance so accidentally current testing is just confusing rather than unsafe.

 

[wallport]

I measured it and got 40.5ma at first (maybe something happening on first connection), dropping to 35.5ma after a couple of minutes. Does this sound alarming?

I can't be sure of course that the draw is the same over a long period.

I tried the calculation but it's all a bit confusing as the drops in voltage are not a straight line and I couldn't find numbers I trusted with actual values rather than estimating from the graphs. If I were to try some new measurements, which period would I use as the voltage seems to fall a lot at first and then settle down and eventually drop very little each 24 hours?

 

[roadtripper]

If you've measured it that's higher quality anyway. If you can arrange it so you can leave it for 20 minutes or so connected you might be able to spot if the ECU on the heater drops into some sort of sleep state like the van ones do.

It's worth checking with the vendor if that's out of spec.

Is it alarming? No that's about what I'd expect from an idle microcontroller but in context it might not be great. The number is small but if it's constant that will consume about 1aH a day.

To put it in perspective I think the VW electronics have an alarm if sleeping current draw for the whole van is over 50ma.

The way you deal with this in control systems is by deep sleep and picking the right microcontroller. Most industrial microcontrollers have the ability to turn off almost all of themselves to save power. Depending on what it's doing some just have a tiny clock that will wake it up every so often to see if it needs to power on again, another popular approach is to have a tiny dedicated low power segment that looks for a button to be pressed.

As with all these things it depends one how much effort the manufacturer wants to put in. It takes time to get that approach right. If you're paying high end for something that wants to be permanently connected I'd absolutely expect it to have a decent sleep implementation. If you are paying more towards the value end I'd hope it would have something as a basic approach isn't too hard.

But at the end of the day if you know what that draw is and it's there 24/7 putting in a decent switch and remembering to turn it off when not needed is mildly less convenient than a well implemented sleep approach, but it works just as well.

I think what generally surprises people is how small drains can make a big impact over time, instinctively folks go looking for much bigger drains.

 

[wallport]

Thanks.

For now I have replied to Autoterm with the readings above and asked them if it's, what they would expect. Let's see what they say and then lll think on the points you make above.