I recently came across the video(
) below in which a pwm charge controller is used as a trickle charger between a lithium battery bank and a LA bank. I'm curious to hear from folks with more technical expertise than me if this a viable solution? Thoughts?
PWM charge controller as trickle charger
- Thread starter pdxbill
- Start date
sunshine_eggo
Happy Breffast!
The issue is that PWM controllers can't effectively modulate current. A battery can deliver GOBS of current. For the stated trickle charge function, it's marginally acceptable.
This really isn't a good idea for normal use. PWM works by shorting PV to battery, and they can't effectively modulate current, so there is a potential for massive surge from the LFP to lead acid. Fuse on both PV and battery wires is critical. The experience with blowing fuses during start illustrates my point.
Rednecktek
Solar Wizard
I connect the load ports to my batteries as a maintainer/charger. It cuts off the charge if the main battery gets too low so the bike and lawnmower don't drain the main system battery down and kill the system.
sunshine_eggo
Happy Breffast!
Load ports are a parallel connection to the main battery. You just have an automatic battery paralleling switch.
Rednecktek
Solar Wizard
I just set the low voltage cut out for 12.6 so it stops charging before the main batt gets too low.Load ports are a parallel connection to the main battery. You just have an automatic battery paralleling switch.
sunshine_eggo
Happy Breffast!
Fused?
Rednecktek
Solar Wizard
Of course!
My charging systems for the bike and lawnmower are both 100w panels to a 10a PWM controller connected to a 35ah AGM battery. The load terminals go from the PWM through an inline fuse to an SAE connector on the bike/mower battery so I can just unplug it when I drive. The low voltage cutoff is set to 12.6 so when weather is krappy the bike/mower doesn't drain down the AGM and hopefully the sun comes out and recharges the main batt. Once the main batt gets to 13v the load ports turn back on.
As far as using an LFP battery to be a fake PV input, most controllers need a few volts over the charging voltage so I doubt there would be enough voltage difference to start the charging process.
My charging systems for the bike and lawnmower are both 100w panels to a 10a PWM controller connected to a 35ah AGM battery. The load terminals go from the PWM through an inline fuse to an SAE connector on the bike/mower battery so I can just unplug it when I drive. The low voltage cutoff is set to 12.6 so when weather is krappy the bike/mower doesn't drain down the AGM and hopefully the sun comes out and recharges the main batt. Once the main batt gets to 13v the load ports turn back on.
As far as using an LFP battery to be a fake PV input, most controllers need a few volts over the charging voltage so I doubt there would be enough voltage difference to start the charging process.
Last edited:
Vigo
Solar Addict
Couple weeks late to this, but i've done this thing a bit. I think the answer is 'yes, but' because the followup question might be more important.. does it actually solve any issues? The assumption would be that if you just hook a ~13.2-13.4v lifepo4 in parallel with a ~12.7v lead acid that you'd waste energy pointlessly raising the lead acid to ~13.2v at all times, but in my experience that doesn't happen and almost no current actually flows between them. At that point you don't really need the voltage setpoints of the SCC to 'protect' the lead from the lithium, because you're in no danger of overcharging it or even really wasting much energy from the lithium side.
That just leaves 'needing' (more like wanting for efficiency reasons) to disconnect the lithium from the lead during lithium charging, and hopefully having a 'low voltage cutout' as well so that in the worst case the lead doesn't take the lithium down with it if there were a short or something on the lead side. You can get an adjustable voltage 'window switch' and trigger a contactor/relay with it, for nearly the same money as a cheap SCC, and it will accomplish this.
So i think using an SCC alone between lifepo4 and lead acid is sort of pointless. If you use it to trigger a larger current-handling switching device based on its internal setpoints it can be a cheap and useful form of adjustable voltage switch, but by itself it does not accomplish much as an upside and has the downside that it will probably smoke during the voltage sag of the lead side during engine cranking (if applicable) because the lithium will try to flow WAY too much current through it during that condition.
That just leaves 'needing' (more like wanting for efficiency reasons) to disconnect the lithium from the lead during lithium charging, and hopefully having a 'low voltage cutout' as well so that in the worst case the lead doesn't take the lithium down with it if there were a short or something on the lead side. You can get an adjustable voltage 'window switch' and trigger a contactor/relay with it, for nearly the same money as a cheap SCC, and it will accomplish this.
So i think using an SCC alone between lifepo4 and lead acid is sort of pointless. If you use it to trigger a larger current-handling switching device based on its internal setpoints it can be a cheap and useful form of adjustable voltage switch, but by itself it does not accomplish much as an upside and has the downside that it will probably smoke during the voltage sag of the lead side during engine cranking (if applicable) because the lithium will try to flow WAY too much current through it during that condition.
Rednecktek
Solar Wizard
The issue there becomes the batteries trying to equalize. Strapping a fully charged 14.2v LFP battery to a 12.7v lead acid the lead acid will try to reach 14.2 to equalize and that's not healthy for the lead.
pollenface
Solar Addict
I do this with a dc-dc converter limited to 150w to supply 52.2v to my AGM 48v bank.
When the sun comes up the 48v is usually in absorption very early in the day.
When the sun comes up the 48v is usually in absorption very early in the day.
Vigo
Solar Addict
In my experience the main complicating factor of paralleling lead and lithium is wanting to get the lithium to 100%. If you dont feel the need to go that high while they are connected then nothing really flows between them even at many tenths of a volt of difference.
sirslayer
New Member
- Joined
- Mar 20, 2021
- Messages
- 73
Use a cheap MPPT controller instead of the PWM solar controller and set the low voltage cutoff , Fuses is an option because some controller have auto protection and should recover the controller from shorts and over loads. The high risk is the wear on the host batteries due to currents can get heavy and it will test your craftsmanship of your wires. I ran a setup similar to this posted thread for over a year and the only grip is that it’s does take a hit on overall efficiency
Ack! SAE connectors. First things I chop off and replace with powerpoles.
I don't know how many high-resistance, poor-contact, and oem polarity reversed versions caused me so much grief when starting out with the HF stuff and auto-parts solar projects back in the day... good times.
At a minimum when I'm lazy with chargers, I'll just make sure that I break the SAE connect once a year, and maybe spray some deoxit on it. Until I can't stand it, chop off and do right...it's too bad, that some chargers from yesteryear were actually quite good, but the SAE connectors were crap and troublesome.
I don't know how many high-resistance, poor-contact, and oem polarity reversed versions caused me so much grief when starting out with the HF stuff and auto-parts solar projects back in the day... good times.
At a minimum when I'm lazy with chargers, I'll just make sure that I break the SAE connect once a year, and maybe spray some deoxit on it. Until I can't stand it, chop off and do right...it's too bad, that some chargers from yesteryear were actually quite good, but the SAE connectors were crap and troublesome.