Charge Controller Tolerances

DIY
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Hey everybody,

I’ve been using a Victorn Multiplus 24V system on my offgrid cabin for almost 4 years now. I’ve loved the modular element of the victron products as I’ve built onto the system as I’ve had money. I started with one 150/100 charge controller, 4 panels, and 2 batteries, now I have 3 150/100 charge controllers (and one 100/30 charge controller that I’m using because I had it from another project, did not intend to mix and match controllers), 24 LFP batteries, 23 panels, and about to add my second multiplus inverter.

My question is regarding design capacity of my charge controllers. I noted that they put off a lot of heat especially if they’re actually pumping out 100A. So I mixed up my panel combiners to limit the current each one of my charge controllers has to manage. However, as they ARE quite expensive, if I want to add more panels to my system, what is a good max Amp charge design tolerance? A tricky question because it only peaks in the middle of the day and sometimes for not very long. But I know from some of my electrician friends that a general rule of electrical sizing in general is that you should design your system to not go above 80% rated capacity under maximum potential load. Is this a good rule for charge controllers as well? Try to limit peak charge current to around 80A for a 150/100? Or am I leaving capacity “on the table”? I also love the idea of having my equipment last longer so I’m happy to buy more units to minimize wear and tear, but I think I’m currently “under utilizing” the charge controllers I do have.

Another aspect of this is that I live in Texas so there are plenty of super bright days but even still my peak current is only for a few hours a day. But during cloudy days I commonly have to run my generator. I’m thinking if I add more solar panels I can get more during the morning and evening hours, and possibly de-rate the charge controller so I limit max charge during peak hours? But if I de-rate my charge controllers with the intention of maximizing functional life span, what’s a good de-rating for? 85-90A for a 150/100? I can get used panels for super cheap and my off-grid home is on 11 acres so I’ve got plenty of space, so I guess what I’m trying to figure out is how to get the most out of the charge controllers I have while balancing their life span.

Thanks to the community. I love my off grid setup and I love I’ve been able to figure so much out on the “university of youtube”. Big shout out to the Explorist.Life channel for all their great videos. I’ve dug around trying to answer this question but if someone has addressed it on youtube or on this forum I haven’t been able to find it.

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You will stress them less by lowering the charge current. I also find they like to be near their max input current vs max voltage. They run a little cooler when the input voltage is closer to the battery voltage. Aim for Vmp ~ 50–80 V that keeps conversion ratio moderate, good efficiency and wiring size reasonable.

I use a mix of MPPT and AC charging on my systems. I have them set to charge during the entire day cycle or use them for solar soakers when solar is at its peak. I use DVCC to adjust the current based on voltage when the battery is full. You can certainly adjust DVCC based on SOC, so when they are near empty, the system will set the max current, and when they are nearly full, the system will lower the current. I can help you with some Node-RED code if you want.

Also, if you plan to put in more solar, start putting in east-west arrays. This gives you more power in the morning and the afternoon and flattens the peak, but gives you the same amount when it’s cloudy.

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Victron’s MPPT units are thermally limited, so will reduce the charge current if they get too hot. I live in the tropics, so routinely have ambient temperatures of 28C. However, I also believe that these units do not have sufficient heatsink area for convective cooling when operating at full current in a warm environment, so I’ve arranged a small fan for each unit to enhance the cooling, and can usually keep the internal temperature to <50C.

So, there is no particular need to ‘derate’ the unit to 80% of it’s capacity - provided there is adequate cooling.

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