Question for a battery expert...

BrentBrent Posts: 64Solar Expert ✭✭✭✭
My home system consists of two Xantrex XW6048 inverters [6 kw ea.] in a grid tied system. The batteries are two parallel 8 battery strings of 6V for a 48V total.

Every morning when the sun comes up the charge controllers go through a bulk/absorption cycle then revert to the float voltage for the rest of the day. The batteries are never discharged except if there is a power failure which happens 3-4 times a year.

My question is do I really need to go through the bulk/absorption cycle every day. The bulk voltage is high enough that the batteries gas a little. I equalize 2-3 times a year.

My goal is to keep the batteries fully charged [and maximize their life cycle] so they will be ready when needed so I am thinking that the float voltage might be sufficient except after a discharge.

Thanks,

Brent

Comments

  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Question for a battery expert...

    Xantrex charge controller, right?

    If it were an Outback or MidNite the Bulk and Absorb stages would be very short because the batteries are being kept up (not cycled) so gassing would be minimal.

    Either way, you lose. Deep cycle batteries are meant to be cycled, not held in eternal Float Mode. They're aging and dying even without being used. You could lower the Bulk/Absorb Voltage to the same level as Float, but it really won't help much.

    This is one of the reasons why hybrid GT systems aren't a good value unless you do have lots of power outages: money invested in equipment that isn't doing anything but aging and dying is not well-spent.
  • NorthGuyNorthGuy Posts: 1,925Solar Expert ✭✭
    Re: Question for a battery expert...

    I am not a battery expert ... :D

    I cycle my batteries quite a bit, but I still work on avoiding absorptions every day. Absorbtions are needed for the batteries after re-charge, but they're harmful - bubblibg, corrosion. If batteries are already full, there's no need for the absorption.

    I would keep them on float during the day (so that you can use solar directly to loads) and at rest during the night. If you have XW SCCs you can do that by programming the bulk and absorption voltages to be equal to your float voltage. If you eliminate absorptions, you may (or may not) need to do equalizatons more often.
  • BrentBrent Posts: 64Solar Expert ✭✭✭✭
    Re: Question for a battery expert...

    Thanks for the response. I did not mention that I do a deep discharge [65-70%] about 4 times/year to exercise the batteries. Just as a note, in my old Xantrex system from many years back I got 15 years out of the Concord battery bank with a similar configuration to the one I have now.

    Brent
  • VicVic Posts: 2,922Solar Expert ✭✭✭✭
    Re: Question for a battery expert...

    Hi Brent,

    Am not a battery expert.

    However, agree that running a charge cycle each day on batteries that have experienced no discharge is not needed and causing some added corrosion to the plates. and some needless water consumption if they are Flooded.

    If it were my battery bank, would cycle the batteries to 55-60% SOC at least once every month, and then fully recharge them.

    Agree that using the battery manufacturer's Float voltage for the daily "recharge" should be fine. Of course, you will need to change the settings for the recharge after cycling the bank manually.

    And you will still want to watch the SGs and water levels (again if Flooded batts). Just my opinions. Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • mtdocmtdoc Posts: 600Solar Expert ✭✭
    Re: Question for a battery expert...

    Are you selling power back to the grid?

    I don't know how it works with XW systems but with my Outback inverter and Midnite CCs I sell excess power to the grid and set the sell voltage at about float voltage.

    Unless there is a grid outage, the batteries are kept at float voltage. Generally for AGMs keeping at float voltage is best for maximum lifespan - flooded batteries are different and as 'coot says need to be cycled periodically.

    The CCs in my system read "bulk stage" during the day while selling despite the fact that in reality the batteries are really being held at float voltage without being discharged below that unless there is a grid outage. When they do occasionally get discharged they go back through a full bulk and absorption cycle before being floated again.
  • BrentBrent Posts: 64Solar Expert ✭✭✭✭
    Re: Question for a battery expert...

    Thanks all for the response. I just talked to a technical guy at US Batteries and he too agreed that it would be best to keep the bulk and absorb the same as the float as the batteries are seldom discharged.

    The US Battery tech said he would expect 10+ years out of the batteries the way I am using them and also mentioned [to my surprise] that it is OK to replace a bad battery with a new one and not replace the string. Something you would not do if the batteries were discharged frequently.

    Brent
  • mtdocmtdoc Posts: 600Solar Expert ✭✭
    Re: Question for a battery expert...
    Brent wrote: »
    Thanks all for the response. I just talked to a technical guy at US Batteries and he too agreed that it would be best to keep the bulk and absorb the same as the float as the batteries are seldom discharged.

    Do you mean the absorb voltage set the same as the float? That is not a good idea IMHO. Once batteries have been discharged with a grid outage or if trying to "exercise" them, they should go through a full absorption stage using the normally recommended absorption voltage which is usually quite a bit higher than the float voltage.
  • BrentBrent Posts: 64Solar Expert ✭✭✭✭
    Re: Question for a battery expert...

    I agree. I would need to reset the bulk and absorb levels after a grid outage to recharge the batteries then set them back to equal the float level. We are out in the country and usually have several power outages in the winter. The battery backup has done a great job keeping us powered up through the outages which can last for several hours to all day.

    Brent
  • NorthGuyNorthGuy Posts: 1,925Solar Expert ✭✭
    Re: Question for a battery expert...
    Brent wrote: »
    I agree. I would need to reset the bulk and absorb levels after a grid outage to recharge the batteries then set them back to equal the float level. We are out in the country and usually have several power outages in the winter. The battery backup has done a great job keeping us powered up through the outages which can last for several hours to all day.

    After outage, XW6048 will automatically run the full cycle charging from the grid (unless you disable grid charger completely). XW6048 has a separate set of settings than SCCs. You can keep normal bulk/absorption/exit settings in XW6048. Since your batteries are only discharged during outage, this will ensure normal charge cycle after every discharge.
    This way, you do not need to swtich settings all the time.

    If you decide to "exercise" batteries by discharging them, you can simply simulate an outage by disconnecting AC1 breaker.
  • BrentBrent Posts: 64Solar Expert ✭✭✭✭
    Re: Question for a battery expert...

    Currently I have the XW6048 charge program disabled so I can do it manually via the charge controllers. Not really a problem. I do simulate an outage by switching off the AC feed so the inverters think the grid went down.

    My biggest problem is knowing when the grid goes down. The transition to battery backup is so seamless that you can't really tell when it happens. I rigged up an alarm in the kitchen that rings when the utility power fails so I don't try and run the oven or dryer when we are on battery power. Works very well.

    Brent
  • mtdocmtdoc Posts: 600Solar Expert ✭✭
    Re: Question for a battery expert...
    Brent wrote: »

    My biggest problem is knowing when the grid goes down. The transition to battery backup is so seamless that you can't really tell when it happens. I rigged up an alarm in the kitchen that rings when the utility power fails so I don't try and run the oven or dryer when we are on battery power. Works very well.

    I can relate. Usually we find out when my wife tries to use the clothes dryer which is not backed up.

    An alarm - now there's an idea- but visual only (?flashing light) - I don't think I want to be awoken at 3 am by a buzzer or bell...
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Question for a battery expert...

    imo no you don't need to go through the full gambit. you can reprogram the controllers for say a bulk/absorb voltage of as little as .1 volt (or a bit higher if you like) above the float voltage, but at float voltage is fine too. this would stop excess gassing and even a few other nasty developments from occurring. now i would recommend that you put the batteries through their paces at least once a month down to at least 75% soc, but no lower than 50% soc and then give them a proper full charge before reverting back to the low charge voltages again.
  • vtmapsvtmaps Posts: 3,738Solar Expert ✭✭✭✭
    Re: Question for a battery expert...
    niel wrote: »
    i would recommend that you put the batteries through their paces at least once a month down to at least 75% soc, but no lower than 50% soc and then give them a proper full charge before reverting back to the low charge voltages again.

    Over on the Wrenches forum a trojan representative has stated that batteries that spend their time floating may stratify and that they should be put through a bulk-absorb charge every week or so to stir up the electrolyte, however he also said that there is no need to discharge them first.

    I mention this, not to be argumentative, but just to point out another perspective. I have no idea which is the better approach.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • NorthGuyNorthGuy Posts: 1,925Solar Expert ✭✭
    Re: Question for a battery expert...
    vtmaps wrote: »
    ... batteries that spend their time floating may stratify ...

    I seriously doubt that. When you dissolve acid in water, this is an exothermic process and heat is generated. To separate them back an equal amout of heat must be applied. Stratification will not happen just because the acid sits there. I had a bucket of sulphuric acid sitting on the basement floor for months. When I checked it with hydrometer taking acid from the bottom and from the top, it didn't stratify even a bit. You need to cycle to stratify.
  • nielniel Posts: 10,311Solar Expert ✭✭✭✭
    Re: Question for a battery expert...

    if you wish to do that full charge every week i'm sure it would be ok on the batteries. i suggest the discharging for other reasons as you can develop problems with constant charging and no discharging and it isn't an exacting thing when we charge our batteries. now the biggest thing with cycling would be a good habit to see to it all functions well together and can be put into a normal maintenance routine.
  • BrentBrent Posts: 64Solar Expert ✭✭✭✭
    Re: Question for a battery expert...

    Thanks again for the comments. From the info I have gathered and from talking to a few battery manufacturers tech people I don't think stratification is an issue. My approach for my battery bank, since it only gets discharged a couple of times/year when the grid goes down, is to keep the float/abs/bulk voltages within a volt of each other [the charge controllers change the float voltage with the battery temp]. Right now the battery temp is at 95F and in December it will be around 45F so the float voltage varies from about 55.16 in the winter to 51.92 in the summer.

    I plan to discharge the batterys [65-70%] every other month followed by a full charge cycle and an equalization every 6 months. This should keep the electrolyte mixed [not sure it is really needed but it won't hurt]. I currently check the SG every few months and top up with water about twice/year.

    Brent
  • westbranchwestbranch Posts: 5,093Solar Expert ✭✭✭✭
    Re: Question for a battery expert...

    HOT! any chance of having the batteries in a cool below ground room?
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • CariboocootCariboocoot Posts: 17,615Banned ✭✭
    Re: Question for a battery expert...
    vtmaps wrote: »
    Over on the Wrenches forum a trojan representative has stated that batteries that spend their time floating may stratify and that they should be put through a bulk-absorb charge every week or so to stir up the electrolyte, however he also said that there is no need to discharge them first.

    I mention this, not to be argumentative, but just to point out another perspective. I have no idea which is the better approach.

    --vtMaps

    I think Trojan needs to hire better people. :p

    Since electrolyte is a solution not a suspension it doesn't stratify without change to the chemical state, and that change should not occur if the battery is kept charged because that charge means the chemical state is consistent.
  • YehoshuaAgapaoYehoshuaAgapao Posts: 280Solar Expert ✭✭
    Re: Question for a battery expert...
    Xantrex charge controller, right?

    If it were an Outback or MidNite the Bulk and Absorb stages would be very short because the batteries are being kept up (not cycled) so gassing would be minimal.

    Either way, you lose. Deep cycle batteries are meant to be cycled, not held in eternal Float Mode. They're aging and dying even without being used. You could lower the Bulk/Absorb Voltage to the same level as Float, but it really won't help much.

    This is one of the reasons why hybrid GT systems aren't a good value unless you do have lots of power outages: money invested in equipment that isn't doing anything but aging and dying is not well-spent.

    Load shifting TOU peak hours to TOU off-peak hours during summer rate periods will give your batteries their cycles while they wait to be truly useful when currency crisis and incompetent government start to impact the power grid. In winter the days are short enough you could just do off-grid days to give your batteries the cycle exercise they need. This is assuming your battery bank is not that terribly undersized.

    My xantrex charge controllers always start a new bulk cycles after awaking from sleep every morning regardless of battery voltage. The load shifting in the tail end of the TOU on-peak rate period 5-8PM at least makes this charge cycle not a waste of battery life.
  • YehoshuaAgapaoYehoshuaAgapao Posts: 280Solar Expert ✭✭
    Re: Question for a battery expert...
    mtdoc wrote: »
    Are you selling power back to the grid?

    I don't know how it works with XW systems but with my Outback inverter and Midnite CCs I sell excess power to the grid and set the sell voltage at about float voltage.

    Unless there is a grid outage, the batteries are kept at float voltage. Generally for AGMs keeping at float voltage is best for maximum lifespan - flooded batteries are different and as 'coot says need to be cycled periodically.

    The CCs in my system read "bulk stage" during the day while selling despite the fact that in reality the batteries are really being held at float voltage without being discharged below that unless there is a grid outage. When they do occasionally get discharged they go back through a full bulk and absorption cycle before being floated again.

    Xantrex has a 'enhanced interactive mode' which essentially is charge controller driven grid support voltage (the voltage the inverter tries to hold while selling to the grid or just powering the subpanel). Bulk-1V during bulk & boost, Absorption-1V during absorption, float-1V during float. My charge controllers are in float most of the day.

    AGMs are better suited for permafloat, but they don't have high temperature tolerance, last half as long, and cost twice as much as flooded. Plus if you have a TOU pricing plan (SRP's is optional but rigged for you to be encouraged to opt-in), loading shifting will provide the flooded batteries the cycles they need.
Sign In or Register to comment.