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Old 08-16-2017, 10:17 AM   #1
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Question DC load off leisure batteries whilst "smart" charger is charging

Hi all, first post of what I'm sure will be quite a few whilst I plan the electrics for my mates T4 van conversion.

So I've done a lot of reading up in the past week and (amongst a load of other things) realised that you should have a proper 3/4 stage "smart" battery charger to get the most out of your leisure batteries - whether that is a B2B from the alternator, a quality solar controller/charger or an AC-DC battery charger. Even better if you get one that you can program to your specific battery. (Yes part of my reading was SolarBob's blog! )

In a simple setup of smart charger connected to battery connected to a DC load, what effect does the load have on the smart charger?

If a have a 100 Ah battery and a 30 A smart charger, with no load it should charge the battery at about 20 A (0.2C). Now what happens if I had add 5 A DC load? Will the smart charger output 25 A (20 A to charge the battery and 5 A to power the load) or will it still only output 20 A thinking it is charging the battery at the full 20 A when in reality it is 15 A to the battery and 5 A to the load?

Any and all input on this appreciated!
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Old 08-18-2017, 08:06 AM   #2
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In reality both, but it really depends on what stage your charger is in. First, I would also challenge your first assumption. I do not believe a 30 amp smart charger is going to automatically find your 0.2C level or 20 amps. I believe the charger will go to its max available charge, which I suspect in your example would be 30 amps. Obviously you have chargers that let you manually set the charge rate.This is important if you would like to purchase a charger for a smaller bank now, but intend to increase capacity in the future. I also expect that their could be smart chargers that could set this automatically, based on a input supplied for the size of your battery bank, I really have not come across one, but admit I haven't looked.

In either case, in the bulk phase is constant current controlled, so the charger will put out the max current available (however that is derived), so if you had a 20 amp charge and a 5 amp load, you still only get 20 amps out of the charger, so only 15 amps to the battery.

Once you meet the voltage threshold, or timeout value on some chargers you then move into absorption phase, which moves from constant current to constant voltage. Voltage is the controlling factor, the current will move to meet it's voltage set point. Remember in this phase the expected current is not targeted at max or (0.2C) rate. Any large current loads would most likely result in the charger going back to the bulk phase.

Moving into the float stage would be similar, the absorption phase, again controlled by a constant voltage. This is again another area where larger current loads can put the charger back into either the absorption or bulk phase. There are a variety of solutions, applied by the various manufactures to handle these kind of issues.

-greg
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Old 08-21-2017, 09:40 AM   #3
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Thanks a lot for your reply Greg - that's all very interesting information and given me a lot to think about.

If the smart chargers are going back into the bulk or absorption stage, due to both load and charger being connected to the battery, won't that over-charge the battery and in turn shorten it's life?

A lot of the wiring diagrams I have seen in blogs of people's van conversions tend to just connect all the different chargers (solar/alternator/mains/etc) and the loads to the battery. The image below, stolen from outbackjoe.com, is an example of such a setup.



Is this a poorly designed system or is that just how it is if you want to charge at the same time a load is connected?

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Originally Posted by Scalf77
There are a variety of solutions, applied by the various manufactures to handle these kind of issues.
Please can I have some examples of the sorts of solutions that are available? Or a nudge in the right direction for what to search to find them myself?

I have seen the CTEK Dual Charger and CTEK Smartpass that work hand in hand to charge the battery from two DC sources and support a load. I've also seen you saying that "CTEK reputation as a battery charger manufacturer is pretty good". However, the Dual Charger only outputs 14.4 V max which is below, for example, the 14.8 V suggested for a daily charge of a Trojan battery (Table 4 - Trojan User Guide). Do you reckon you could use the CTEK Smartpass with a different B2B charger (like a Sterling) that you can program correctly for the specific type of battery?
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Old 09-19-2017, 10:44 AM   #4
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Originally Posted by OutdoorAcorn View Post

If the smart chargers are going back into the bulk or absorption stage, due to both load and charger being connected to the battery, won't that over-charge the battery and in turn shorten it's life?
No.....part of the current supplied by the charger goes to battery charging and part goes to the load. If the battery is topped up.....then very little current (or none) is flowing to the battery and the charger is just providing current to the load.

A battery monitor will show you exactly where the juice is flowing.

The basic concept here is that for the most part, the current flowing to a load is determined by the resistance of the load....so your 20A or 30A charger will only deliver 5A to a load that needs 5A based on it's resistance.
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Old 09-19-2017, 07:45 PM   #5
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Originally Posted by OutdoorAcorn View Post
Thanks a lot for your reply Greg - that's all very interesting information and given me a lot to think about.

If the smart chargers are going back into the bulk or absorption stage, due to both load and charger being connected to the battery, won't that over-charge the battery and in turn shorten it's life?

A lot of the wiring diagrams I have seen in blogs of people's van conversions tend to just connect all the different chargers (solar/alternator/mains/etc) and the loads to the battery. The image below, stolen from outbackjoe.com, is an example of such a setup.



Is this a poorly designed system or is that just how it is if you want to charge at the same time a load is connected?


Please can I have some examples of the sorts of solutions that are available? Or a nudge in the right direction for what to search to find them myself?

I have seen the CTEK Dual Charger and CTEK Smartpass that work hand in hand to charge the battery from two DC sources and support a load. I've also seen you saying that "CTEK reputation as a battery charger manufacturer is pretty good". However, the Dual Charger only outputs 14.4 V max which is below, for example, the 14.8 V suggested for a daily charge of a Trojan battery (Table 4 - Trojan User Guide). Do you reckon you could use the CTEK Smartpass with a different B2B charger (like a Sterling) that you can program correctly for the specific type of battery?

Question for any of you -- For storing a SMB around a month at a time would it be best to use a smart charger or just plug into shore power? My SMB is an 06 E350 with (2) two starting batteries and one large AGM house battery. Thanks for any input.
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Old 09-19-2017, 09:23 PM   #6
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I'm a fan of low amp trickle charging. A multi stage low amp charger is what I use on my bass boat and the AGM batteries have lasted years. I have solar on the van so I don't need to apply a high amp charge the inverter puts out very often. The only issue I've had using my particular shore charger was when one of the starting batteries cells shorted. Spewed acid/water all over the place and got really hot. But I had all kinds of clues telling me something was wrong. Now I kinda know what to look for.
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Old 09-19-2017, 09:38 PM   #7
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I'm a fan of low amp trickle charging. A multi stage low amp charger is what I use on my bass boat and the AGM batteries have lasted years. I have solar on the van so I don't need to apply a high amp charge the inverter puts out very often. The only issue I've had using my particular shore charger was when one of the starting batteries cells shorted. Spewed acid/water all over the place and got really hot. But I had all kinds of clues telling me something was wrong. Now I kinda know what to look for.
Daveb - Thanks for the response. My SMB will be covered during storage, thus the solar will not be able to kick into charge. In this case do I still use a "low amp" trickle charger? If so, any recommendations on a brand or type? I looked on line and there seem to be dozens of different types, sizes, ratings etc.

Thanks - this is all somewhat new to me.
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Old 09-19-2017, 10:27 PM   #8
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I'm a fan of low amp trickle charging...
The only issue I've had using my particular shore charger was when one of the starting batteries cells shorted. Spewed acid/water all over the place and got really hot. But I had all kinds of clues telling me something was wrong. Now I kinda know what to look for.
I'd be interested in knowing what those clues were prior to the 'explosion'.

And did that occur while using the low amp trickle charger? Or while charging from a higher amperage source?

Thanks!
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Old 09-20-2017, 06:14 AM   #9
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No.....part of the current supplied by the charger goes to battery charging and part goes to the load. If the battery is topped up.....then very little current (or none) is flowing to the battery and the charger is just providing current to the load.

A battery monitor will show you exactly where the juice is flowing.

The basic concept here is that for the most part, the current flowing to a load is determined by the resistance of the load....so your 20A or 30A charger will only deliver 5A to a load that needs 5A based on it's resistance.
Thanks for your reply boywonder. That has helped me clear up a few things in my head but still unsure about some stuff!

I get that if the charger is in float mode (constant voltage but a lower voltage than when in the absorption stage) then the charger will only supply the current that is needed to keep the battery topped up plus any current the load is drawing.

However, like Scalff77 said previously, if a large current load is applied and the battery charger goes from float back into absorption then the voltage will increase. Isn't it bad for the battery if it is subjected to the higher voltage of the absorption stage for a long period of time?

If the battery charger goes from float back to bulk (which is constant current) will the battery still only draw the current that is needed to keep it topped up or will the charger push the full 20/30A (minus the load) to the battery? And what will the voltage be when in the bulk stage with a fully charged battery? Sorry I'm not really sure how constant current works.
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Old 09-20-2017, 07:32 AM   #10
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Originally Posted by Botchan View Post
Daveb - Thanks for the response. My SMB will be covered during storage, thus the solar will not be able to kick into charge. In this case do I still use a "low amp" trickle charger? If so, any recommendations on a brand or type? I looked on line and there seem to be dozens of different types, sizes, ratings etc.

Thanks - this is all somewhat new to me.
I'm using a 2/4/8A Battery Minder on the boat for a maintenance charge. I've also have a smaller CTEK charger I use on single batteries. I'm sure the marine industry folks have some really nice lower amp chargers as well.



Quote:
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I'd be interested in knowing what those clues were prior to the 'explosion'.

And did that occur while using the low amp trickle charger? Or while charging from a higher amperage source?

Thanks!
Kind of dovetails off the OP's question. As one of the batteries failed I was having some starting issues. And the float level of the house batteries would bleed off quickly when the charge was stopped...that was new. Then there was the fact that the charge current would not drop to its normal fully charged point of an amp or two with everything shut off. Stupidly I fired up the inverter/charger over night to give the batteries a good hard charge. In the morning I smelled the off gassing battery and looked at the inverter control panel I was shocked to see it was putting out just shy of 50amps. The bad battery was so hot I couldn't touch it. It did not overcharge the other 3 batteries. Actually I gave the other starting battery to a buddy and he used it for several years. The "smart" charger just saw the bad battery as load and kept supplying all it could to it. Now if I had used a low amp charger, it wouldn't have been able to keep up. The separator would have opened as the voltage dropped and the bad battery would have surely killed the other in a short period of time. Right now my inverter supplies light maintenance charge when plugged even with the charger disabled. It's just enough to keep the batteries at about +/- 13 volts. The separator never opens throughout the night. The only large load is the fridge and while on shore power it runs off AC and not the batteries. This has worked for me over the years.
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