24V Battery bank connection questions

rmcewen
rmcewen Registered Users Posts: 8 ✭✭
I am going to install a new battery bank for my 24V solar system.  The batteries consist of 6 SLR125 12V VMaxTanks batteries. So I will have three sets of two batteries each wired in series to get 24V and then wired in parallel which should produce 375AH.

My first question concerns creating a system as balanced as possible. As a reference we can use the methods described on this page but hopefully my descriptions will suffice:

http://www.smartgauge.co.uk/batt_con.html

1. The first and most obvious method is to wire the six batteries in a serial-parallel arrangement and use the opposite corners for the connection to the rest of the system.  This is method 2 shown on the referenced web page.  According to the web page this results in a good but not perfectly balanced system.  I believe this is the most common method of connection.

2. The second possibility is to wire each individual serial wired two battery (24V) bank to common bus bars with equal length wires.  This is shown in "method 3" of the referenced web page.  In this case, the bank should be perfectly balanced as each pair of batteries is connected individually to the bus bars which provide the connection points to the rest of the system.  The only downside I see to this approach is the extra length of wire needed to connect each pair to the bus bars.  In my particular case, this would be about 30" of 1/0 wire.  I would have six 30" lengths connecting to the bus bars vs. two 30" lengths.

If it matters, the maximum charge and load amperage would be about 35A.

So is the second method worth it to achieve perfect balancing given the extra lengths of wire required?  Disregard any cost issue as I already have the wire for either method.

My second question concerns low SOC disconnect.  The reason I'm installing a new battery bank is because I destroyed my old bank by letting the batteries fully discharge.  This happened due to a loose connection on an MNEPV breaker resulting in no charge getting to the batteries for several days.  I assume the inverter shut down properly at low voltage but I also have several DC loads which continued to drain the batteries until they were dead.  How do most people protect against this type of occurrence?  How is the low SOC detected and then how are the DC loads disconnected to prevent the batteries from losing any more charge?  In my situation the system is in an off-grid cabin which I don't reside in permanently so am not onsite to monitor this type of thing manually.  Several of the DC loads are things like a 3G modem for internet connection, security cameras, refrigerator, etc. so need to remain running full time so just turning off all loads when I am not present is not an option.  I'm just wondering what most people do to prevent this kind of thing.  

Thanks.

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,433 admin
    For your first question--Either will work fine. Your 1/O wire is way heavier than you need for 35 amp loads--But should be fine.

    I suggest that you get a DC Current Clamp Meter (DMM from Sears is "good enough" for our needs).

    Regarding shut down of AC inverter... The low battery cutoff by most AC inverters is ~10.5 (12 volt) or 21v (for 24 volt battery bank).

    The low voltage cutoff for an AC inverter is really there to protect the AC inverter--As the battery voltage drops, the DC input current increases for the inverter (power=voltage*current--As the voltage falls, the input current must increase to keep constant power to the AC loads).

    To protect your battery bank, a voltage of ~11.5/23.0 volts can help--Note that lead acid battery output voltage is highly variable--And a fixed LVCO set point is not really that accurate (suggest battery maximum discharge is around 50% to 20% state of charge--The deeper the discharge, the more damage/shorter life the battery will have). Usually, only more expensive AC inverters have a programmable LVCO setting.

    For DC loads, you could probably find a Battery Cutoff switch like this one (I know nothing about product or website--Just an example):

    https://www.batterystuff.com/battery-products/switches/BG-40.html

    You can get a Battery Monitor (either current/shunt based, or voltage based) unit so you can monitor battery capacity when you are there. Also, there are a few that have an "alarm contact" you could use to notify you when your battery capacity is low (one may have a data bus, or there are several Internet enabled solar charge controllers with remote monitoring).

    A problem with trying to "harden" your solar power system--The more sophisticated the system, the more complexity and greater chances that something else could go wrong.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rmcewen
    rmcewen Registered Users Posts: 8 ✭✭
    Thanks for the reply.  Getting a DC clamp meter is a good idea.  I'll definitely do that and check the charge/discharge currents of each battery individually.

    As for the low voltage disconnect, I just wasn't sure how useful a voltage based solution would work given the variable voltage under load like you mentioned.  I have both a Trimetric and a Whizbang Jr. In theory I can get a reasonable SOC value from the Whizbang Jr. over modbus and trigger a shutdown when SOC reaches a certain threshold.

    I have the Midnite Classic w/ Whizbang Jr hooked up to an ARM board running linux (Odroid U3) that controls my security cameras and DC relays and uploads data to the web.  So adding a software trigger to shutdown everything based on SOC shouldn't be too hard.  My inverter has a remote shutoff also so I can shut it off based on SOC and not rely on its LVCO since you point out that isn't good to protect the batteries.

    You're right about complexity though.  I thought I had everything covered but didn't count on a loose wire from the solar panels.

    I was mainly just curious how most people handled this type of situation.  Maybe most people just shut everything off when absent for weeks at a time.  

    Thanks for your advice Bill.