backchannel: isolator confusion

My claim:  “When the batteries are deeply discharged a plain solenoid will typically provide as much current as a 225Ah bank can accept (much more than 300w of solar can make at solar noon). And it does it for 1/10th the cost of the solar.”

I mentioned the 300w:225Ah and solenoid configuration because that’s what s/he has on their van.

It isn’t recommended to use a solenoid as an isolator

Charging linked batteries is the same with any non-b2b isolator.

if you’re using different battery types/sizes as simultaneously charging batteries of different size or age is not good for them.

The danger from isolator charging is if the house battery is intolerant of alternator voltages.  This could be an issue with barefoot (no BMS) lithium house banks, particularly with non-LFP 3S lipo that is fully charged in the mid-twelves.

Mismatched batteries are deprecated within house banks for reasons unrelated to charging.

> I installed a switch to disconnect the house batteries from the vehicle battery so I am only using it when necessary.

This unnecessarily hamstrings the isolator’s contributions to the overall system.

> You obviously have no idea how charging batteries works


> as most alternators cap at 25A

What?  Why would an alternator (probably 145A in the case of OP’s Express cargo van) cap current to a house bank at 25A?  How would it know the demand was coming from another battery attached to the system?

Why are isolators typically manufactured with  75A-200A  ratings if the alternator will only provide 25A?

Lead batteries pull current;  it is not pushed into them.   They take what they want.  And 220Ah of deeply-cycled lead wants much more than 25A.

> to avoid overchargingboiling

This is a relative non-issue at alternator voltages.

(just like any quality charger),

Chargers are rated at 25A (for example) because it costs more to build them to higher spec.  They use staged charged to charge the batteries properly with minimal outgassing.

Whether you’re at 300W or 600W, you can’t charge a battery for a 30 minute commute and expect it to be done. Even an hour isn’t enough to charge house batteries.

I didn’t claim any of that.   I have included my claim at the top of this post for convenient reference.

One method to keep from making shit up is

  1. quote the material you want to address

  2. then address it.

> You wouldn’t run your vehicle for 8 hours to give your batteries a good charge.

You’re right;  I wouldn’t do that.  I would do what the post is about, add isolator charging to an existing solar install.   Perhaps we could stick to the topic at hand.

It’s more effective at topping off daily use which you don’t necessarily need if you have solar.

No, it’s more effective at Bulk charging, which is where solar is weakest.   It’s particularly _in_effective for “topping off”.

> Isolators do help for people who drive a lot,

They help for people who drive when the house battery voltage is lower than alternator voltage.

> and i recommend one if you have the money but the charge you get doesn’t even compare to solar when you consider the amount of time the vehicle is running.

What does it mean that the charging from isolator “doesn’t compare”?  Luckily there are units of measurement that make comparison quite clear.

This is the math, but it’s easily verifiable in the real world.

  • 300w of panel on MPPT (our respondent’s setup)  will make about 255w at solar noon under good conditions (5% mppt losses, 10% panel heat derating).  At 12.1v (50% DoD, our “deeply discharged” scenario) that is 21A for ~$420, or $20-per-Amp.

  • 300w of panel on PWM will make about 202w under the same circumstances (12.1v x ~18A ISC).  That is (an optimistic) 18A for ~$350, or ~$19.45-per-Amp.

  • an average cargo van with a $50 solenoid setup will feed a 225Ah bank  to current saturation while driving.  For SLA that would be 45A (C/5), $1.11-per-Amp.  For AGM it would be 75A (C/3), $0.67-per-Amp.  Even if we accepted OP’s imaginary 25A limit that would still be $2-per-Amp.

Yes, this bang-for-buck only lasts a short while until Vbatt == Valt, at which point current starts dropping rapidly in a kind of quasi-Absorption.  That’s why we combine isolator and solar charging.  And it’s why I qualified my claim the way I did.