Solar float mode: Necessary?

[Snowman9000]

As a practical matter, is float mode necessary with solar charging? As an alternative, the controller would simply stop charging when the battery was full. The next day the process would repeat.

Floating was developed to maintain batteries over weeks and months of continuous charging. Solar charging is not continuous, and the cycle restarts each (sunny) day.

I wonder if solar float mode is "because we can and the customer thinks they need it", rather than because it produces any real benefit.

Couldn't the extra watts put in at float voltage be instead put in at absorption voltage? Amps would be low; charge pulses would be short. Is there float in an automotive alternator charge?

[Almot]

Snowman9000 wrote:
If using a controller w/o float, if you drew AH from the fully charged battery and there was still sunlight, I would expect the charge to (resume in order to) keep up. Albeit at 14.4v, not 13.2.

Yes, it would resume if battery or loads would demand. But you don't want to keep fully charged battery at 14.4V.

[BFL13]

Snowman9000 wrote:
If using a controller w/o float, if you drew AH from the fully charged battery and there was still sunlight, I would expect the charge to (resume in order to) keep up. Albeit at 14.4v, not 13.2.

The controller is kicked back into "Boost" when the battery voltage gets down to 13.2. You could run some loads late afternoon and only get down to 13.3v. Of course there is loaded voltage being lower, so it also would depend on how big the load is.

[BFL13]

smkettner wrote:
IMO float is good in all conditions. Most alternators drop voltage with time based on heat or are controlled by the main ECM.

Does "all conditions" include when you will be doing an "incomplete recharge" by dropping to float, when you could have reached "full" before dark if you had stayed at Vabs?

[Snowman9000]

If using a controller w/o float, if you drew AH from the fully charged battery and there was still sunlight, I would expect the charge to (resume in order to) keep up. Albeit at 14.4v, not 13.2.

[time2roll]

IMO float is good in all conditions. Most alternators drop voltage with time based on heat or are controlled by the main ECM.

[BFL13]

"Charging it fully in Bulk and Absorb mode without Float would mean unnecessary heating/boiling - it's better to let it finish the last 5-10% in Float."

It is better but not always possible on solar if dark gets there first. It could be necessary to leave it at the higher Vabs till the batts are full, and then drop to float or as the OP suggests let night-time do that anyway.

IE it is more important to get to full than drop to float at 90% and never reach full that day.

It is all situational. Each "user" needs to keep an eye on what his situation is at the time and place and adapt. Having a Float option on the controller lets you adapt to more situations.

[Almot]

Snowman, Float can be disabled in higher-end controllers. I think my Rouge MPPT has such an option, don't remember since this wouldn't be a good idea EXCEPT when you have Li battery. Li batts need no floating when full.

All other batteries do need float mode, and not only in storage. Float helps saving battery cycling in afternoon when battery is full and solar can run the loads "directly", with almost all the current going to loads and only a fraction of current - to counter self-discharge. As mentioned by others, a good controller will automatically switch back to Bulk (or MPPT, whatever) if the loads draw more than controller provides in Float mode.

Charging it fully in Bulk and Absorb mode without Float would mean unnecessary heating/boiling - it's better to let it finish the last 5-10% in Float.

[brulaz]

Snowman9000 wrote:
As a practical matter, is float mode necessary with solar charging? As an alternative, the controller would simply stop charging when the battery was full. The next day the process would repeat.

Floating was developed to maintain batteries over weeks and months of continuous charging. Solar charging is not continuous, and the cycle restarts each (sunny) day.

I wonder if solar float mode is "because we can and the customer thinks they need it", rather than because it produces any real benefit.

Couldn't the extra watts put in at float voltage be instead put in at absorption voltage? Amps would be low; charge pulses would be short. Is there float in an automotive alternator charge?

Think I understand where you are coming from. When not using the batteries, I drop the solar controller's float down to 13.1 for the usual reasons. And like you suggest, I could probably turn it off completely without any ill effects as every morning the batteries would get full charged.

But when boondocking and using the batts during the day, I would rather have the batteries charged up as much as possible before nightfall. So I set float quite high at 13.6V or so. If battery V falls below (13.6-0.5) V, the solar controller moves into MPPT mode if there's sunlight.

13.6V float is much lower than the absorption voltage (14.8V), but as long as the controller can maintain it, I figure the batteries are in good enough shape for the evening.

[azrving]

X2 what Golden said. Especially when I'm charged by 11:00 am or noon. The rest of the day would be negative ah. I suppose you could turn the float down if you have the ability.

In fact my Bogart Trimetric has a read out that says FULL and they suggest running heavier loads such as vacuum etc during that time of day as the battery is full and being maintained yet there is excess power available.

Of course it doesn't have to be Bogart product to utilize this energy management concept.

[Golden_HVAC]

When I was living in my RV, and had a E-meter that counts the number of amp hours going out of the battery and back in, I could tell my energy use each hour, day, and when it was full again.

So say I was at -90 amp hours at the end of the day, and it started recharging with solar. Around 2 pm, the battery would be full, but I might still be consuming 5 amp hours for small loads. If the solar just 'stopped' until the next day, then I might be using up say 20 amp hours while it is still sunny out, and that would all come from the battery, not the solar panels that are sitting there 'shut off'.

By running at 13.4 volts once the battery is full, it loweres the amount of water boiled off (should it had been allowed to run above 14 volts) and prevents the battery from getting to warm. But it still allows all energy being consumed while the sun is still out to be solar powered, not drawing from the battery while there is still plenty of sunlight hitting the panels.

Anyway it is nice that the solar controller does not stop completely once the battery reaches 13.3 volts, but it will cycle back on if you are drawing some power from the battery.

Have fun camping!

Fred.

[pianotuna]

Float mode is good for the longevity of the battery.

[BFL13]

It is mostly for when using the solar to maintain full batteries when you are not camping in the RV.

Also, when camping you can be in a situation where you get to Absorption Voltage (Vabs) early in the day, and you spend a long time at Vabs before dark, while the batteries "lose water." In that case you might want to reduce to a float voltage after a shorter time at Vabs