Mixing Panels in a MPPT setup

lazza · November 2011

Hi Forum

no time to even read all the replies to subscribed threads, a shame, such interesting stuff.

Anyway this query regards another bodged system. It has 5 existing panels (anonymous make... hence rubbish) of about 75W, and another panel of another strange make, of about the same size. They are all put in a 12V system and i reckon the Isc of each panel is about 5A.

The job is to make the system work better. I was considering installing two more panels either Bauer 24V at 5.35A Isc, or KC120 12V at 7.5 A Isc. Question:

Can you mix panels of different current ratings as well as different voltages with an MPPT charge controller---- what are the do's and dont's for MPPT??

Thanks as ever,

Larry

niel · November 2011

Re: Mixing Panels in a MPPT setup

no problem in reiterating it as it can't be said enough i guess.

when paralleling pvs you want to try and match the voltages and when putting them in series you want to try and match the current. now it does not need to be exactly the same in the matching, but the farther away you go the worse it will be. i like to recommend no more than a 5% excursion and others say 10%, but these are arbitrary in light of my previous sentence.

Cariboocoot · November 2011

Re: Mixing Panels in a MPPT setup

When it comes to mixing panels of different specs the type of charge controller doesn't matter; the rules (-of-thumb) are the same.

Without the panel specs you're shooting in the dark - and bound to hit your own foot.

lazza · November 2011

Re: Mixing Panels in a MPPT setup

Ok, so it's the same basic rules for mixing panels for MPPT as for fixed nominal voltage settings?

Cariboocoot · November 2011

Re: Mixing Panels in a MPPT setup

lazza wrote: »

Ok, so it's the same basic rules for mixing panels for MPPT as for fixed nominal voltage settings?

Yes; no charge controller can make up for differences in the panels.
(Now someone will find one somewhere with multiple panel inputs each with its own MPPT.

)

lazza · November 2011

Re: Mixing Panels in a MPPT setup

Ok thanks.

When adding to these existing installation, it seems the best bet to put in our panels with a separate charge controller and all the necessary protections. The added advantage of this is we can identify any problems further down the line, and demonstrate that it's not due to our setup

.

Now is the other "no-no"... mixing batteries. The installation currently has 3, 200Ah 12V Gel batteries all in parallel, that seem to be in a good state of charge and to have considerable life left.

Really another battery of around 200Ah is required to ensure this particular client doesnt have to resort to the generator as frequently as they do now. Of course I know that mixing batteries is bad practice and that having more than 2 parallel strings is also problematic. But to keep costs reasonable, there seems to be no alternative but to add a battery with similar characteristics (we cant find the existing make)... maybe we could add a 180Ah battery to compensate for the fact that the existing batteries are 3 years old?? any bright ideas/pointers most welcome

Thanks
Larry

ps I have considered switching the setup to 24V, but with the dodgy panels and their irregular currents.. it could make things worse i feel

Cariboocoot · November 2011

Re: Mixing Panels in a MPPT setup

This is doomed to failure.

First, you can not mix different types of batteries regardless of their Voltage/capacity. Gel cells are the worst choice for RE.

Second, even if you use the same type and spec battery you do not know how much capacity has been lost in the originals. Guessing that it's 20 Amp hours is just that; a guess. Measuring the remaining capacity can be done, but it isn't easy and probably won't help.

The bottom line is whenever you put two or more batteries together that do not have the same specifications for whatever reason, all the batteries involved will suffer for it. It's just throwing money away.

P.S.; Putting the new panels on their own controller is a really good idea. You can't do any more than that to ensure best utilization.

lazza · November 2011

Re: Mixing Panels in a MPPT setup

Thanks very much for the reply.

Well, the big question therefore would be- is it better to leave the 600Ah system as it is, or transform it to 800Ah system by adding a battery ( unfortunately we have no option but for it to be gel, as the other 3 are gel), risking the deterioration of the batteries?. Which option will result in more energy for the next say, 3 years?

Larry

ps I know it sounds like we should be strung up for bad practice, but the only business right now is fixing bad workmanship of previous setups... we're just trying to make these systems work for people.:-)

mike95490 · November 2011

Re: Mixing Panels in a MPPT setup

lazza wrote: »

..., or transform it to 800Ah system by adding a battery ( unfortunately we have no option but for it to be gel, as the other 3 are gel), risking the deterioration of the batteries?. Which option will result in more energy for the next say, 3 years?....

GEL batteries are not a good choice for off-grid household use. AGM or flooded batteries are prefered. GEL has poor high current performance, discharge and charge. It's very easy to put too much charge into them, and create bubbles at the plates, which never go away, and reduce capacity.

Tell your client they will need new batteries, and to start saving for them. And to run the generator as needed, maybe get a new, small AutoThrottle InverterGen to use to top off batteries daily. It's WASTEFUL to add fresh batteries to a damaged system. When they can't stand it any longer, install a new bank.

But - be sure to do a power survey before you buy any more gear, is the current system the right size, too small, or too large?

BB. · November 2011

Re: Mixing Panels in a MPPT setup

What is their daily kWH average load?

I think many installers and owners, in the olden days (a few years ago) got in the habit of recommending more batteries for "undersized" systems--When in reality, they had undersized charging sources for their daily loads.

Unless they are discharging the existing bank more than 50% in 24 hours, and/or they have a string of days with cloudy weather--I would suggest your plan of adding more solar panels + charge controller is really the best right now.

If you suspect that their existing solar panels/charge controller are of questionable power output/reliability... I would warn them that they may have some work ahead on their existing solar array. And then once your 1/2 is up and running, take a DC current clamp and (hopefully) show that your XXX watt system is at 80% of rated output (Vmp*Imp, Vbatt*Ibatt) and the other system is at 50%.

I would hate to see your 1/2 working well, but have an unhappy customer that does not know their older hardware is deteriorating and/or not productive. And the new components are operating correctly.

If you can get them enough "reliable" solar power--Perhaps that will reduce their generator runtime enough to make for a happy customer.

-Bill

PS: To add to Mike's comment... Is the genset properly sized for power needed? I.e., what is the kWH per liter of fuel they are getting today (large genset running at low output levels to charge a too small AC charger and/or too small/low current acceptance of battery bank (using the C/20 or 5% recommended charging current for GEL).

Cariboocoot · November 2011

Re: Mixing Panels in a MPPT setup

lazza wrote: »

ps I know it sounds like we should be strung up for bad practice, but the only business right now is fixing bad workmanship of previous setups... we're just trying to make these systems work for people.:-)

Larry; you and I do the same thing - fix other people's mistakes. Most of the initial inspections I do involve screaming.

If you can get some that are close to the originals and don't cost too much (HA!), add the batteries: three years they should last anyway. Just make sure the client understands that a total bank replacement is in the future.

lazza · November 2011

Re: Mixing Panels in a MPPT setup

HI All, thanks for your replies

Yes, i have to try very hard not to let out swear words when revising these systems.

As for kWh consumption- they only really consume alot at the weekends, it's a weekend cottage really. But there is a fridge consuming throughout the week. We have put an Efergy energy monitor there to measure its consumption, because without that info it's very difficult to know if the panels alone will provide the necessary extra energy to the batteries, or if we need more Ah. If the fridge consumption is low (unlikely) then maybe the panels alone will suffice.. but if its more than 1kWh per day, I reckon extra capacity will be needed.

As for the generator.. they only put it on at the weekends now and again, but they'd rather not have to put it on at all... it's a manually operated system, and the idea is to remove the need for it more or less completely.

The existing system is giving about half its rated power-- which with rubbish mixed panels, inclined at 60º! (I didnt know Spain was in the arctic circle), with the steel frame covering some of the cells on 2 of the panels, disconnected by-pass diodes (we will solder them back on) ... just shows the resilience of PV systems.

solar_dave · November 2011

Re: Mixing Panels in a MPPT setup

Don't forget load shifting. If your panel add gets the batteries in shape before the end of the production day, then run some elective loads against the system to give the panels some work to do.

lazza · November 2011

Re: Mixing Panels in a MPPT setup

Yes that would be ideal.. but how do you get loads to react according to SOC of the batteries? The inverter is a cheap piece of junk, so no way to use the inverter for that...the charge controller is a Steca 3030 and dont think that has this option either.... any suggestions?

Mixing Panels in a MPPT setup

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