Setting absorption time on my MPPT

[blackrose]

The default absorption time on my MPPT (for flooded lead acid batteries) was set at 180 mins based I assume on the data I entered on the size of the battery bank and looking online at how that's calculated the formula seems to be 0.42 X C/20 / charge current (where 0.42 is assumed current losses while in absorption mode and C/20 is the 20 hour Ah rating of the battery bank).

 

So for my 450Ah bank with a maximum charge current of 55A from my MPPT the absorption charge works out to about 3.4 hours.

 

However, are these absorption times meant to be for daily charging? On shore power my batteries used to sit on float and perhaps get a once a week cycle from the battery charger, so I'm just a bit uncomfortable with such a long daily absorption charges at 14.8v so I've reduced it on the MPPT to 80 mins, but have I made a mistake?

 

The reason I'm asking is that I don't want to overcharge or undercharge them. They're still getting fully charged with the reduced absorption time according to my battery monitor - depending on the amount of sunshine of course.

[rusty69]

13 hours ago, blackrose said:

 

However, are these absorption times meant to be for daily charging? On shore power my batteries used to sit on float and perhaps get a once a week cycle from the battery charger, so I'm just a bit uncomfortable with such a long daily absorption charges at 14.8v so I've reduced it on the MPPT to 80 mins, but have I made a mistake?

 

 

The batteries should ideally be fully charged after every use. Whether that is daily, weekly or monthly will depend on how you use them. 

 

13 hours ago, blackrose said:

 

The reason I'm asking is that I don't want to overcharge or undercharge them. They're still getting fully charged with the reduced absorption time according to my battery monitor - depending on the amount of sunshine of course.

 

Unless you are overcharging them, and losing a lot of electrolyte to gassing, and need to top up with water a lot, I can't see a problem with the 3 hour absorption time. I think some Trojan batteries specify a maximum of 4hrs absorption.

 

Obviously if you undercharge them, sulfation will occur,leading to reduced capacity, and as you are using solar, you may be in danger of not fully charging them every day on duller days. 

To achieve this happy medium using solar alone will mean changing the MPPT setting on a regular basis, although I suppose you could give it a blast once a week with a mains charger (or engine). I guess you have done a specific gravity test?

The definitive answer,of course, will lie with Trojan technical dept.

[blackrose]

Thanks Richard, I haven't done a SG test recently but the batteries have been performing well. I was down in the engine hole recently and could hear them bubbling away so I just thought that perhaps the MPPT was overdoing it, but perhaps I'm worrying too much. The other reason I reduced the absorption time was because they seemed to be using more water than before I installed solar because of the daily cycles.

 

I removed the equalisation charge from the MPPT program when I first installed it because I didn't want it doing that on a daily basis and I don't have the option to reduce the frequency of equalisation charges on my MPPT. 

 

Perhaps I'll increase the absorption time up to 2 hours.

Edited July 10, 2023 by blackrose

[blackrose]

Does anyone know if absorption times on MPPT controllers are cumulative? 

 

What I mean is, if you have absorption time set for say 3 hours but the sun is going behind clouds and coming out again every 10 minutes like today so the voltage fluctuates, does the controller add up all the absorption voltage times from when the sun was out to get the total of 3 hours?

 

Those with victron MPPT controllers should be able to see this. I can't because I bought an Epever Tracer which doesn't give you much charge history.

[DaveP]

On Epever (mine's a xtra 4215), it appears to be a simple timer counting down from when the batteries hit the absorbtion voltage on charge.  That said, I've got Lithiums and the timer is only 10mins @ 14.5v - but even when the sun goes in with a minute or so of them hitting 14.5 and the voltage drops away, it still switches to float mode once the timer's up.

[blackrose]

Ok thanks. That's a very short absorption time but I know nothing about lithium batteries.

 

So if a cloud came over 5 mins into your absorption cycle, would the Epever make up the other 5 mins later when the sun came out and it was able to achieve 14.5v, or would it start the 10 min cycle again from the beginning?

 

I'm asking because my absorption time is 3 hours at 14.8v. The MPPT is only able to reach 14.8v when the sun is shining so I'm trying to work out whether it just adds up all those bits of absorption charging until it adds up to 180 mins?

Edited July 17, 2023 by blackrose

[DaveP]

2 hours ago, blackrose said:

Ok thanks. That's a very short absorption time but I know nothing about lithium batteries.

 

So if a cloud came over 5 mins into your absorption cycle, would the Epever make up the other 5 mins later when the sun came out and it was able to achieve 14.5v, or would it start the 10 min cycle again from the beginning?

 

I'm asking because my absorption time is 3 hours at 14.8v. The MPPT is only able to reach 14.8v when the sun is shining so I'm trying to work out whether it just adds up all those bits of absorption charging until it adds up to 180 mins?

No, it just counts to 10, says 'job done' whatever happened to the voltage and switches to float. No saving up of time below 14.5v.  It can be provoked into resetting the timer by pulling a large current out (1kW kettle or the like for a few minutes) but definitely not by merely hiding the sun away....

Edited July 17, 2023 by DaveP

[blackrose]

6 minutes ago, DaveP said:

No, it just counts to 10, says 'job done' whatever happened to the voltage and switches to float. No saving up of time below 14.5v.  It can be provoked into resetting the timer by pulling a large current out (1kW kettle or the like for a few minutes) but definitely not by merely hiding the sun away....

 

Ok thanks but I'm not sure I understand? I wasn't asking about saving up time below 14.5v, I was talking about whether the timer adds up all the bits of time at the absorption voltage - above 14.5v?

[DaveP]

7 minutes ago, blackrose said:

 

Ok thanks but I'm not sure I understand? I wasn't asking about saving up time below 14.5v, I was talking about whether the timer adds up all the bits of time at the absorption voltage - above 14.5v?

Nope, hits 14.5v, starts timer, doesn't care about voltage, counts to 10, drops to float.

[blackrose]

Ok thanks. I'm not sure but I don't think that's what's happening with mine. 

Edited July 17, 2023 by blackrose