I need more power Scotty!

[Vic Harder]

So I'm looking at ~60 ah a day max and will more likely be in the ~50 ah per day range most of the time. I'd like my system to run for 2 days without any driving or charging, so ideally I'd like a battery in the ~110 ah range.

Are you already assuming lithium in this statement? Because with LA don't you want 2x your max drawdown available, so that you don't run below 50% most days? That would mean 200 AH is what you should spec for?

That's what I assumed for my setup, and your power need calculations and desired 2 day interval are the same as mine.

 

[Esus]

Are you already assuming lithium in this statement? Because with LA don't you want 2x your max drawdown available, so that you don't run below 50% most days? That would mean 200 AH is what you should spec for?

That's what I assumed for my setup, and your power need calculations and desired 2 day interval are the same as mine.

Good catch. I should have had it read: ideally I'd like a battery in the 100 ah "usable" range.

Also, I kept typing 110 when the battery is 100 ah. I'm going to edit my post for clarity.

 

[rando]

Did Battle Ready provide you with a real spec sheet for their battery with thermal limits etc? I couldn't find a detailed one on their web site. In my searching for a LiFePO4 BMS, I have not found one that had a low temperature cut off for charging, so it would be slick if they had one.

Edit to add - it looks like these batteries only have one set of terminals, so I am not sure how the BMS could cut off battery charging at 32F without entirely disconnecting the battery, which would make it useless in the winter. This would definitely something you should get explicit confirmation from the manufacturer about. It would suck to spend $900 on a battery that you won't be able to use for much of the year.

 

[Esus]

Did Battle Ready provide you with a real spec sheet for their battery with thermal limits etc? I couldn't find a detailed one on their web site. In my searching for a LiFePO4 BMS, I have not found one that had a low temperature cut off for charging, so it would be slick if they had one.

Edit to add - it looks like these batteries only have one set of terminals, so I am not sure how the BMS could cut off battery charging at 32F without entirely disconnecting the battery, which would make it useless in the winter. This would definitely something you should get explicit confirmation from the manufacturer about. It would suck to spend $900 on a battery that you won't be able to use for much of the year.

That would really suck! Just called to find out. Here' the scoop from their engineer:

Within the battery there are separate pathways for charging and discharging. At 20-25°F it will stop taking a charge but will still allow discharge. At 131°F it will stop charging AND discharging.

 

[baphenatem]

I would agree with most of this. The current is limited to 15A on both input and output. The 75/15 is rated for 440w of solar though, meaning that at the max 15A of input current the PV array would have to be hitting 29V.

How does this play out practically? Let's say you put 3 of those 100w panels on the roof in series (specs? likely something like max voltage of 20V and max current of 7 amps) your max combined voltage in series is 60v and at 7 amps (Series current doesn't add - Thanks Rando for pointing out my error here), and applying Rando's 75% rule of thumb = 45V at 5.25A or about 236 watts.

The controller will try to give the battery a specific voltage, depending on how you have configured the controller, but for argument's sake, say about .5V more than it is currently sitting. If the battery was at 13.2v, (close to fully charged for AGM's) then that's 13.7V. 236/13.7 = 17A. In my case, I am using a controller limited to 15A (not sure on that... see my question in the post two down) so I won't see that, and I am "wasting" those 2A or about 28 watts.

I think.

A note on this: Don't connect three of the "12 volt" 36 cell modules in series and connect them to the Victron 75/15. The open circuit voltage on most modules is about 22.5V at 25degC, which puts you at 67.5V for the string. If the temperature drops below 0degC the voltage will exceed the 75V rating of the 75/15. You could go with a 100/15 and have plenty of leeway.

 

[Vic Harder]

good point. I'm wondering if the device has the same kind of over voltage protection as it does over current?

 

[baphenatem]

good point. I'm wondering if the device has the same kind of over voltage protection as it does over current?

I don't believe that it does. There are a few others, such as Midnite Solar, that have an overvoltage allowance, but they simply don't allow the unit to operate during the time when the voltage is above the maximum operating parameter.

 

[Vic Harder]

On the other hand, the manual does specifically state that it can be used with 3 panels in series. Not saying one should do this, but it is within spec?

"The controller can be used with any PV configuration that satisfies the three above mentioned conditions. For example:

12V battery and mono- or polycristalline panels

● Minimum number of cells in series: 36 (12V panel).
● Recommended number of cells for highest controller efficiency: 72 (2x 12V panel in series or 1x 24V panel).
● Maximum: 108 cells (3x 12V panel in series)."

 

[Esus]

On the other hand, the manual does specifically state that it can be used with 3 panels in series. Not saying one should do this, but it is within spec?

"The controller can be used with any PV configuration that satisfies the three above mentioned conditions. For example:

12V battery and mono- or polycristalline panels

● Minimum number of cells in series: 36 (12V panel).
● Recommended number of cells for highest controller efficiency: 72 (2x 12V panel in series or 1x 24V panel).
● Maximum: 108 cells (3x 12V panel in series)."

Makes me wonder if I over did it with the 100/30 for my 290 watt, 3 panel setup that's in the mail as we speak.

 

[Vic Harder]

What is this "over did" it you speak of?

 

[Esus]

What is this "over did" it you speak of?

Ha! Better over than under I suppose!

 

[baphenatem]

On the other hand, the manual does specifically state that it can be used with 3 panels in series. Not saying one should do this, but it is within spec?

"The controller can be used with any PV configuration that satisfies the three above mentioned conditions. For example:

12V battery and mono- or polycristalline panels

● Minimum number of cells in series: 36 (12V panel).
● Recommended number of cells for highest controller efficiency: 72 (2x 12V panel in series or 1x 24V panel).
● Maximum: 108 cells (3x 12V panel in series)."

It's simply a function of temperature. The 108 cell setup will exceed 75 volts if the modules get down to about freezing. If they stay above that, no problem.

 

[badtoytrd]

Can anyone answer one quick question about the Victron 75/15? It it required to have a load connected our can you just run it to the battery/batteries. I have a National Luna kit in my Tacoma with one aux battery under my hood and one battery in the camper connected in parallel. I am looking to charge those off of the 75/15.

 

[rando]

You don't need to use the load terminals - you can just use the battery terminals.

 

[badtoytrd]

Thanks. That's what I figured but just wanted to confirm.

 

[badtoytrd]

Quick question about the Victron Bvm. I am running 2 Odyssey PC1400s in parallel and each battery is rated at 60-65ah for a total of 120-130ah total but I read that only 50% (60-65ah) of that is useable. Can anyone confirm that? I want to ensure that I set the BVM up correctly and that it is not giving me an incorrect percentage.

 

[rando]

This is an interesting point - the full 120 - 130 Ah in 'useable' and is what you should program into your BMV as the capacity. However, the greater the capacity you use from the batteries the shorter their lifetime. The 50% limit is in no way a rule or hard limit, it is entirely up to you how much battery capacity you want to use and how many cycles you expect your batteries to last for. There is a great graph from Lifeline that I have posted before that illustrates this point:

I personally have no problem going down to 70% Depth of discharge (or 30% State of charge) on a regular basis, and would even go down to 80% - 90% on occasion (although I have had no call to go that low yet). My camper is relatively new, but so far I have had about 30 nights in it (mostly in the winter), and according to my BMV-700 this has resulted in only 12 charge cycles. At this rate, even if I were to take it regularly down to 80% depth of discharge every single time, the battery would still last around 1000 nights of camping, which would be fine by me.

 

[badtoytrd]

Rando

I knew you would come through. That is the exact info I was looking for. I had seen the graph in early posts but just want to make sure I was programming properly. Thanks again.

 

[badtoytrd]

Got my Victron 700 BVM installed tonight and I couldn't believe how easy it was. I still need to shorten the comm cable to the display but man it was an easy install. Thanks to all of those that answered my questions. Now onto my MPPT 75/15 install this weekend.

 

[Esus]

Congrats! I like "easy." I have the same setup installed but haven't programmed yet. Got the Bluetooth adapter so I'm hoping to use my iphone to program the BVM 700 and my MPPT 100/30 this weekend!