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Axpert MK 5 grid tie with out batteries


Pieter Annandale
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Welcome to the forum Pieter.

 

 

You will find a wealth of experience and advice here so ask all those questions that need answering and there are a dozen or so chaps who are passionate about solar and will be able to give you good solid advice.

 

Unfortunately the Axpert needs to be connected to batteries. Depending on the model there may be a minimum number and size of battery needed for your battery bank.

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i had and idea that would be interesting to test..  and that is to get 4 small cheap UPS batteries for about R100 each and connect them to my inverter.  Set the unit to 80% DOD.. just to sort of con the inverter to startup and then have enought solar panels to keep from using batteries that much... but as the batteries are cheap why worry if you kill them.. 

 

so is a idea, and would basially be the same as a grid tie system..

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Well, for one, you don't want to charge at more than 10% of the Ah capacity, and you don't really want to discharge at more than 20% of capacity. Now as these UPS/alarm system/gate/garage-door-opener batteries are 7Ah or thereabouts, that means charge at 700mA and discharge at no more than 1.4A. So that means charge at 30W for charging, and 65W discharge max. Some MPPTs can't even do such low values. My BlueSolar 75/150 only starts charging when there is around 1 amp incoming from the panels.

 

So you will likely physically inflate those batteries like balloons on charging (if you don't push them into thermal runaway and blow them up first), and it will have very little capacity for doing anything.

 

I know of only one legit reason to connect an inverter to such a small bank: If you only need to power the inverter for programming or other debugging. When the firmware on my Victron was flashed recently, the technician used a small 18Ah bank to power it (because the Victron runs off battery power when flashing or doing other programming -- not the grid).

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Someone at work also wanted to connect the smallest possible battery bank to the Axpert to make use of PV mostly. Since the 3KVA and up Axpert models cannot charge the batteries at less than 20A, the smallest battery bank you can actually connect to it would be a 175Ah (20h) bank. That is charging at between 10% and +-13% of the 20 hour amp-hour rating. I would not recommend anything smaller than 170Ah - definitely not 7Ah.

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Someone at work also wanted to connect the smallest possible battery bank to the Axpert to make use of PV mostly. Since the 3KVA and up Axpert models cannot charge the batteries at less than 20A, the smallest battery bank you can actually connect to it would be a 175Ah (20h) bank. That is charging at between 10% and +-13% of the 20 hour amp-hour rating. I would not recommend anything smaller than 170Ah - definitely not 7Ah.

Scary thing is... lots of people are using these with al-cheapo 100Ah batteries :-)

 

I predict lots of Axpert owners looking for batteries in about 18 months :-)

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I predict lots of Axpert owners looking for batteries in about 18 months :-)

Less. Maintenance free 'deep cycle' batts do NOT last ... been there, done that.

During load shedding this year, 2 x 105ah (24v) batts lasted about 20-30 x 80% DOD cycles on a UPS before one batt gave up. Both had to go.

Load was +-700w powered for +-2.5 hours at a time.

 

BUT, going to back the idea that Vader4d (the force is stirring in this one) is asking about.

 

Why not get cheapish 2nd hand batteries that you KNOW are not going to last, and learn using them first?

Maybe someone here or someone knows someone else who has a brother ... that has a few batts that can be used for a few weeks?

I mean, the time is now to play, being holidays and all!

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Less. Maintenance free 'deep cycle' batts do NOT last ... been there, done that.

During load shedding this year, 2 x 105ah (24v) batts lasted about 20-30 x 80% DOD cycles on a UPS before one batt gave up. 

 

HI TTT 

 

As with all things it depends on the jockey. I have 4 x 260Ah CB Solar maintenance free deep cycle batteries. Although I have only had them just under six months, (definitely not long enough to be definitive) they seem to be in better shape than when I received them. They track each other well now under charge and I no longer have a large difference between their voltages during absorb. I have invested fairly heavily in hardware to monitor and protect them (the equivalent of a 5th battery). At a daily DOD of 70% they are due to last about 4 years. I might still be in for a nasty surprise but I charge at C13 and think I treat them with care.

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Why not get cheapish 2nd hand batteries that you KNOW are not going to last, and learn using them first?

Maybe someone here or someone knows someone else who has a brother ... that has a few batts that can be used for a few weeks?

I mean, the time is now to play, being holidays and all!

 

Depends where in the world he lives. I have 4 old Trojan T125 batteries here. Of course that's not enough to get him started, he's going to need 4 more. I'm willing to sell these 4 at scrap value, which I believe is around R250 a piece. So R1000 for 4 batteries.

 

They are 5 years old. One of them has a bad cell, it works well enough, but the hydrometer only registers about a SG of 1125. The other batteries all measure okay on the hydrometer, but one of them takes this dive-off-a-cliff. They are still perfectly good for 2 hours load shedding (at around 350W load).

 

Maybe TTT has a few more old T105s... ? :-)

 

Edit: Also, I'm leaving for Guateng tomorrow... so likely not much use offering it now.

post-157-0-80377700-1450380960_thumb.jpg

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