Battery cables & balancers

[Jeza]

Hi,

I'm in the process of putting together a multi-mode hybrid PV system at our house in New Zealand.  I've been reading a number or forums and have had some advice from a few people but wanted opinions on a few items around battery cable size recommendations and battery balancing.  The intention of the system is to allow load shifting away from peak pricing periods (usually 5pm - 9pm) and to incorporate charging our Nissan Leaf from PV when sufficient generation is available.  The system will be grid tied, though ideally will be configured to avoid grid feed in as my current electricity provider doesn't yet support payment for feed in.  Excess generation would be diverted to variable rate EV charging, water heating & battery charging.  The household loads will be spit to move lighting and some outlets to the backup side of the inverter.  Our off peak pricing is fairly cheap so battery charging would be scheduled every night to ensure batteries are brought back up to 100% SOC on a daily basis.

I've opted for a Goodwe 5048D-ES inverter, 14x Trina Honey Mplus 295w Mono PERC modules and initially will run with some AGM batteries with the intention to move to lithium or lead carbon sometime down the track.  Currently I have 16x Powerware PWHR12500W4FR 124 Ah (C20) 12v batteries that I have been given.  They've done a few years in a UPS system, resting voltages look good at between 12.93 -13.00 V after being off the charge for four days.  Testing with a light load shows no significant volt drop so none of the batteries appear to have dead cells.  I know it's not an ideal configuration, but for now I will be running 4 parallel strings of 4 batteries in series, as per the attached image (but with four strings).  Cable lengths for each series string will be the same and each string will be individually protected by a DC MCB or fuse.

The inverter has a maximum charge / discharge rate of 100A so somewhere around 25A per string if things are balanced.  My main questions are what cable size would people recommend for the battery links in each string, and what cable size between the inverter and battery bank.  The links between batteries only need to be 15cm or so.  The cable run from the common point of the battery banks to the inverter would be 3 - 4 meters.  I can source pre-made 16mm2 30cm cables (crimped & soldered), which might be an option for the battery links.  I was thinking 25mm2 between the battery bank and inverter.  What would the recommendation be for individual string fusing / MCB sizes?

I'm also considering throwing some battery balancers in the mix and was looking at the HA-02's, 1 per series string.  Are battery balancers beneficial with an inverter than can run an equalisation cycle?

Any advice would be much appreciated

 

Thanks,

Jeremy

[Guest]

Hi Jeza. Welcome.

There are calcs you can use to get the exact size cable required based on volts / amps and length required. This in turn dictates the fuse needed.

Here is my take. After a few years of cables, calculating them, I ended up with flexible busbars between the batteries. I got tired of having to think of the max amps so I went and be done with it. 

Cables from inverter to battery is 25mm2 in my case, if it was not part and parcel of the inverter, I would have installed 50mm2 cables.

Because the cables are over sized, the fuses are smaller in that they will go long before the cables will even heat up. Fuses I specced a little bit above the max amps the inverter will draw on flat batteries.

[malek94]

for 200ah battery i used  80mm2 cable is that right ?

[Chris Hobson]

6 hours ago, malek94 said:

for 200ah battery i used  80mm2 cable is that right ?

The battery cable size is determined by the max draw of the inverter. The fusing for that cable is also dependant on the inverter draw.

 

[___]

10 hours ago, malek94 said:

for 200ah battery i used  80mm2 cable is that right ?

Assuming it's lead acid, you probably don't want to discharge that at more than 50 amps, so 35mm^2 would have been fine (depending on length, overall voltage drop and so on). Then you'd fuse it at some value that is higher than the maximum draw of the inverter, but lower than the maximum of the cable, probably around 100 amps in this case.