Charging house batteries

richard5933 · September 18, 2018, 08:53:58 AM

Quote from: thomasinnv on September 18, 2018, 08:46:07 AM
If it were me i would go with a big boy relay and a bi-directional charging relay. When either bank is being charged (ie. Running the engine or being plugged into shore power) the relay will automatically engage and charge the other bank. You can buy one with or without an auxiliary start feature built in. (It's really easy to just wire in a manual switch yourself without that option.) The relay control will automatically engage and disengage at preset voltage levels to prevent accidental discharge of the wrong bank.

What happens if the two battery banks are at very different state of charge. Wouldn't the battery bank with the higher state of charge end up being over-charged? Or, would the high state of charge of the one bank cause the regulator to reduce the charge prematurely? I've always liked my house batteries to have a multi-stage charger connected to them - would the regulator in the bus work the same way?

windtrader · September 18, 2018, 10:15:36 AM

It all seemed so simple until my mind got all twisted up in this thread. My setup and thinking goes like this, please tell me I'm ok or an idiot.

Setup: Single heavy duty high current manual switch between house and start battery banks.
Use:
> If the house bank needs a charge and bus is on the go, I connect them so the bus alternator charges both banks.
> When parked and boondocking, disconnect the banks to protect the start battery from unnecessary discharge.
> If start battery is low, connect together to give starting an extra boost from the house current. > Backup plan - connect battery banks, start generator, enable inverter/charger mode, take a nap or go have a coffee. Start bus.
> Backup plan part 2 - generator battery is dead, SOL.

Jim Blackwood · September 18, 2018, 10:30:47 AM

That's a lot like the manual boat switch since you can use it to bridge the banks at any time.

Jim

richard5933 · September 18, 2018, 11:24:59 AM

Quote from: windtrader on September 18, 2018, 10:15:36 AM
It all seemed so simple until my mind got all twisted up in this thread. My setup and thinking goes like this, please tell me I'm ok or an idiot.

Setup: Single heavy duty high current manual switch between house and start battery banks.
Use:
> If the house bank needs a charge and bus is on the go, I connect them so the bus alternator charges both banks.
> When parked and boondocking, disconnect the banks to protect the start battery from unnecessary discharge.
> If start battery is low, connect together to give starting an extra boost from the house current. > Backup plan - connect battery banks, start generator, enable inverter/charger mode, take a nap or go have a coffee. Start bus.
> Backup plan part 2 - generator battery is dead, SOL.

That would certainly work and allow charging of house batteries on the road. We went with a more complex system because I wanted to have a multi-stage charger on the house batteries which monitored them separately from the chassis batteries and charged them accordingly.

oltrunt · September 18, 2018, 07:26:03 PM

I'm quite enamored with the Sterling BB 1230 battery to battery charger and am about one key stroke from buying it. My only hesitation is that I'm not sure that the system can keep my start batteries from over charging. I can't seem to find definitive assurance that this won't happen. Jack

richard5933 · September 19, 2018, 03:44:43 AM

Quote from: oltrunt on September 18, 2018, 07:26:03 PM
I'm quite enamored with the Sterling BB 1230 battery to battery charger and am about one key stroke from buying it. My only hesitation is that I'm not sure that the system can keep my start batteries from over charging. I can't seem to find definitive assurance that this won't happen. Jack

When you say 'keep my start batteries from over charging' I assume you mean that you're worried about depleting your chassis start batteries while trying to charge your house batteries.

There are a few ways to connect these to prevent that from happening.

1 - Out of the box they are set to pull current from the chassis system only when the alternator is putting out current and charging.

2 - You can set the Sterling to come on manually with a switch on the dash that you flip when you want to charge the house batteries.

3 - You can connect a lead to the Sterling which is powered only when the chassis alternator is charging and use that to trigger the charge on the house batteries.

All that said, you've got to do the math to be sure that your alternator output is adequate to power your current needs PLUS the 30-amp draw that is possible from the Sterling. Of course, this caveat will apply in any situation you use. The Sterling will only pull its full 30 amps when your house batteries are depleted. But so will any method you use to connect your house batteries to your chassis system - if your house batteries are depleted enough to draw 30 amps they will.

oltrunt · September 19, 2018, 06:27:21 AM

Thanks Richard. It is my understanding that a fully charged battery can be damaged when given continuous high charge such as when the charger is trying to charge a very low (or damaged) battery. It is that I am trying to avoid. Jack

richard5933 · September 19, 2018, 07:03:32 AM

Quote from: oltrunt on September 19, 2018, 06:27:21 AM
Thanks Richard. It is my understanding that a fully charged battery can be damaged when given continuous high charge such as when the charger is trying to charge a very low (or damaged) battery. It is that I am trying to avoid. Jack

The Sterling battery-to-battery chargers do not supply any charge to the chassis start batteries, so there is no way that damage to them could be done unless something goes drastically wrong. The charging circuit in the Sterling is multi-stage and will adjust the voltage as needed to keep from roasting the house batteries. It is possible to set custom voltage levels for the stages if the pre-set ones don't work for you. Be prepared to sit and study the manual for quite a while. It's not terribly intuitive, and it's packed with so much information that it is not a quick read.

oltrunt · September 19, 2018, 08:55:39 AM

You're right about the manual not being very intuitive but I see no reason why it should be. I've read it several times and as far as it goes it makes sense. I've also studied all of the videos offered by Sterling and they too make sense. The thing I'm having trouble with is that the start battery is in the flow of the alternator output at all times whether or not it is in need of a charge and regardless of the charge rate set by the BBC for the house batteries. When the BBC is demanding high alternator output, the start battery receives that alternator output on its way to the device. I'm probably overthinking this but i'd still like a definitive answer. Jack

sledhead · September 19, 2018, 09:36:55 AM

this will not help but I have 2 like this right at the top of my dash on the left corner . 1 for coach and 1 for start batteries .
I can see them from outside and I leave them on at all times so I can see the voltage at any time

https://www.amazon.ca/DROK-DC2-5-30V-Waterproof-Digital-Voltmeter/dp/B00CK3P20S/ref=sr_1_9?ie=UTF8&qid=1537374774&sr=8-9&keywords=12+volt+led+meter

dave

sledhead · September 19, 2018, 09:38:09 AM

dave

richard5933 · September 19, 2018, 09:47:21 AM

Quote from: oltrunt on September 19, 2018, 08:55:39 AM
... The thing I'm having trouble with is that the start battery is in the flow of the alternator output at all times whether or not it is in need of a charge and regardless of the charge rate set by the BBC for the house batteries. When the BBC is demanding high alternator output, the start battery receives that alternator output on its way to the device. I'm probably overthinking this but i'd still like a definitive answer. Jack

How would that be any different from turning on any other load in the coach's electrical system? The coach's alternator & regulator will just see it as a load and adjust accordingly. I had the same problem overthinking this too, until I started to just picture the BBC as a load on the system and nothing more.

You turn on the bus headlights, and suddenly there is a large load. The regulator & alternator adjust. Turn on the OTR a/c and there is a huge load. The r & a adjust. Turn on the BBC charger with its 30-amp load, and the r& a adjust. If the r & a are working properly, they should adjust to suit the increased load and nothing more. Coach batteries shouldn't notice any difference from any other load on the system.

oltrunt · September 19, 2018, 10:11:25 AM

"I had the same problem overthinking this too, until I started to just picture the BBC as a load on the system and nothing more." Hum---- that's a good way to look at it.

Dave. Two meters like you have would certainly give me my "definitive answer".

Thanks, Jack

richard5933 · September 19, 2018, 10:35:32 AM

We've got a Victron battery monitor on our house system. It has a shunt on the negative post of the house battery bank, and all current to/from the house batteries flow through this and are monitored.

Watching the readout has been educational. If the 120v charger is turned on and I'm in storage/maintain mode, the voltage on the house system will be 13.2v and the current flow through the battery will be zero or nearly so. If I turn on a 12v appliance the current will go up to reflect the new load on the batteries. Within seconds of it going up, it will again start going back to zero as the charger increases its output to compensate. It's a very well-coordinated dance between the loads and the charger. The voltage will stay the same the entire time (and will depend on what mode the charger is in.)

Why do I mention all this? Because my suspicion is that if you put the voltmeter in either of the circuits it may not provide the information you're looking for. It's possible for the current flow to increase but the voltage to remain the same. My 120v house battery charger (PD9270) will adjust the mode according to the state of charge of the batteries. The voltage on the house battery side when the BBC is running should be whatever the particular mode is set for (bulk, float, maintain, etc) in the BBC, and the mode will likely adjust itself as needed to accomplish proper battery charging/maintenance.

I'm not at all an expert on the inner workings of a bus voltage regulator as it works in these situations. Does the voltage always go up when it needs to supply more current flow? My suspicion is that as long as the chassis batteries have a good state of charge the current flow will increase but the voltage will stay the same. If the batteries were partially depleted the voltage would go up AND the current flow to the batteries would increase regardless of the external load on the batteries.

Maybe someone with more understanding of the regulators on a bus can confirm my theory, that the voltage output to the batteries increases as the voltage level (state of charge) of the batteries is decreased.

thomasinnv · September 19, 2018, 11:29:52 AM

Quote from: richard5933 on September 18, 2018, 08:53:58 AM
What happens if the two battery banks are at very different state of charge. Wouldn't the battery bank with the higher state of charge end up being over-charged? Or, would the high state of charge of the one bank cause the regulator to reduce the charge prematurely? I've always liked my house batteries to have a multi-stage charger connected to them - would the regulator in the bus work the same way?

The bank with the lowest soc is going to absorb the bulk of the charging current. As the voltage rises the amperage ansorbed goes down. If you were to place an amp meter between the alternator and the battery you would find that a fully charged battery will be "pulling" just a few amps at say 13.5v whereas a heavily discharged battery will be pulling close to the rated output of the alternator at the same voltage. The amount of amperage a battery will absorb is related to the soc of the battery and the charging voltage being applied. The voltage regulator connected to your alternator is just that, a VOLTAGE regulator. Technically, the charging amperage is not limited, except for the output limitation of the alternator or as a factor of the output voltage in relation to the soc.

The type of system that I mentioned before is found in probably 99% of stick and staple diesel pusher rv's on the road. Simple, inexpensive, and works very well. No sense in over thinking things or trying to reinvent the wheel.