Navitron Renewable Energy and Sustainability Forum

Energy/Electricity Storage and Use/Grid Connection => Off-Grid, Batteries & Inverters => Topic started by: billi on January 16, 2011, 10:22:24 AM



Title: Battery as a dump load
Post by: billi on January 16, 2011, 10:22:24 AM
Can one connect a second battery  (same voltage ) to the  main Battery  as a dump load ?

Would there be too many amps flowing or too little ? Assume thats hard to say

But somehow it would make sense  to have a cheap second battery charged

Billi


Title: Re: Battery as a dump load
Post by: clivejo on January 16, 2011, 10:30:36 AM
What happens when the second battery is full?  Wouldn't it be better connecting the two batteries in parallel and keep the dump load as a fail-safe as its designed to do?


Title: Re: Battery as a dump load
Post by: billi on January 16, 2011, 10:34:01 AM
the idea is to start a cheap inverter then ,  with a voltage controlled relay   to divert   power to AC consumer(s)

Its not only for a Windturbine , more or less to avoid PV not pushing all available power into batteries when in absorption/float mode


Title: Re: Battery as a dump load
Post by: Billy on January 16, 2011, 10:46:01 AM
billi, billy here.

I thought about that.  Seemed a waste to be dumping all that lovely power only to assist global warming.   facepalm

My idea was to spit the batteries in two and always have a low bank that needed charging.  Trouble is I was going to kill the batteries in a rather short time.  You could end up with 180 cycles a year.

Off grid I reckon the amount of energy actually converted for my use is quite small, probably less than 25% so I am keen to make the best use of it.

What are you doing with all this power you are making?  Not brewing I hope.

billy

 ;D ;D ;D


Title: Re: Battery as a dump load
Post by: clivejo on January 16, 2011, 10:49:24 AM
I think you would need to be careful how the dump load operates.  If it just connects in-line with a relay you could risk huge current flows and burn out your cables.  For example if you have a fully charged battery and connect it to an empty battery, current will flow from the full battery to the empty in an attempt to equal out, this will involve a large current flowing between them.

Dump loads tend to have calculated resistance, in order to safely disperse any extra power being generated.  With batteries resistance varies with charge level and voltage, making it very unpredictable.

Is there no way of starting the inverter slightly below your dump load threshold ?  This way the inverter load should prevent the dump load kicking in, but also keeps your batteries fully charged?


Title: Re: Battery as a dump load
Post by: billi on January 16, 2011, 11:11:25 AM
only distilling  Billy   exhappy:

Our consumption  went up a fair bit , since my 3 year old knows how to control all the gear  whistle and much more washing  machine use

But beside this  it is always good to know  to have 4 or 5 kwh stored in a spare battery   and if needed just hook it into the victrons   and charge the main battery

Clivejo  true  and i would like to find out  how much current we are talking about , but the charge controller wil stop that current when the main battery gets below voltage setpoints

To use my relay in the inverter would be an alternative , but not a permanent one cause will not allow my chargecontroller to optimal charge the main bank

Billi


Title: Re: Battery as a dump load
Post by: clivejo on January 16, 2011, 11:23:52 AM
Do you have a main inverter?  You could set a high battery threshold to trigger a high load device (ie immersion heater)  this way you are making the most of the energy you are generating.

Also most batteries don't like being 'empty' for extended periods of time.  By regularly leaving a battery empty it will reduce its life.


Title: Re: Battery as a dump load
Post by: clivejo on January 16, 2011, 11:33:09 AM
only distilling  Billy   exhappy:

Clivejo  true  and i would like to find out  how much current we are talking about , but the charge controller wil stop that current when the main battery gets below voltage setpoints


Depends on the capacity of both your battery banks and the difference in the level of charge, also the type of battery.  But its more than likely to do damage, either damage the wiring due to over current or your batteries due to over charging/discharging.  A high charge or discharge will damage plates in wet batteries


Title: Re: Battery as a dump load
Post by: Justme on January 16, 2011, 12:48:37 PM
Downsides

need to keep bat SOC low so it can accept a dump load, so extended periods of low SOC will kill the battery
The splitting of the bank into two lowers total available capacity
The battery will not provide a constant load whilst in dump mode
Eventually you will get a time when the bank is full & you still need a second dump load
Excessive water used due to over charging
battery self discharge will be more of an issue if you have long periods of no charging the dump battery.


Title: Re: Battery as a dump load
Post by: billi on January 16, 2011, 01:16:20 PM
Ok it is perhaps too much of an non calculable  experiment  ,  so perhaps back to my ancient  idea of using a DC pump and pump water in my stored hydro lake  ;D to harvest the energy later , but switching a pump constantly on is not good either  whistle , or like clivejo says use my AC relay from the Inverter to charge a battery  or run a AC pump

 has anyone an idea where to get good Solid state relays  like this in Europe

http://www.power-io.com/library/databulletin/hdd-e-family.pdf

Billi


Title: Re: Battery as a dump load
Post by: EccentricAnomaly on January 16, 2011, 01:56:39 PM
need to keep bat SOC low so it can accept a dump load, so extended periods of low SOC will kill the battery

...if it's lead-acid.


Title: Re: Battery as a dump load
Post by: Justme on January 16, 2011, 02:01:33 PM
need to keep bat SOC low so it can accept a dump load, so extended periods of low SOC will kill the battery

...if it's lead-acid.

True, but that is all that is affordable at this time.


Title: Re: Battery as a dump load
Post by: biff on January 16, 2011, 02:57:00 PM
hi folks,
      before i set up my forklift battery bank i had a bank of 10 x 2 100ah 12 volt yousa, this was an exellent bank but i always thought i was abusing it when we used the washing machine,so thats really why i installed the forklift batts,
     my installation set up included a lever type fuse box that consisted of 3 x 20 amp fuses which pushed into place with considerable effort, lifting the lever knocked out the inverter which was connected to one end of all the fuses,the fuses themselves permenantly  connected the controller to the batteries,so having one fuse in place could have the forklift batts in use,then i push in the other fuse which connected up the yousa bank and then withdraw the fuse for the forklift bank, this way i could swop over the banks in seconds, this was a good way to keep the batteries charged up, leaving in the 2 fuses resulted in the forklift batteries pulling down the charge in the yousa because the yousa had a much higher voltage.
     its been a trouble free set up,apart from one forklift pack(72volt) running a bit dry,which was cured by a glorious tipple of rain water,the 48 volt pack was fine,a total teetotaler,,,,,,,,, so this is one way you could have a second batterybank.
                                                 biff


Title: Re: Battery as a dump load
Post by: Outtasight on January 16, 2011, 05:07:38 PM
I'm considering something for a secondary battery pack that can be run flat (NiMH or Lithium). 

I've got some old NiCad 4Ah cells (12V worth) that I might make up into a pack and use with a tiny 70W inverter for running just CFL lights on.  That would spare running the main 3kW inverter that draws 35W when idle!  NiCads don't mind running flat.  They actually need it to stop developing the memory effect.

I can also get 12V 4.5Ah NiMH laptop battery packs for my ancient Toshiba laptop very cheaply (25).  These have the thermistors built in for charge safety already so could be charged from a solar charger quite easily.


Title: Re: Battery as a dump load
Post by: Justme on January 16, 2011, 07:18:43 PM

I've got some old NiCad 4Ah cells
I can also get 12V 4.5Ah NiMH laptop battery packs for my ancient Toshiba laptop very cheaply (25). 

Billi would need a fair few of them to use as a dump load for his 1600watt turbine.

Like 750 worth for the second hand laptop packs for just one hours worth of dumping or 1.6kWh.


Title: Re: Battery as a dump load
Post by: billi on January 16, 2011, 07:39:35 PM
Quote
Depends on the capacity of both your battery banks and the difference in the level of charge, also the type of battery.  But its more than likely to do damage, either damage the wiring due to over current or your batteries due to over charging/discharging.  A high charge or discharge will damage plates in wet batteries

sorry just went for a pint and remembered my intention  ;D

So if we compare two batteries   with two ponds ( had only 2.5 pints  ;) )  the main pond fed with max  2000 watt  the second  pond/battery  connected to the main  one ,

 the main pond starts to be full at 29 volt  but flow rate is  still  1800  watt to grant the 29 volt for full charge , so 200 watt has to go  , in a PV setup  its just wasted before the pond overflows to the second pond

If we allow a relay to open a valve to let the water /watt flow to the second pond , that then would mean the second battery would get a decent (perhaps too much ) of a charge for a very short while  simultaneously  the water-level (volt) of the first pond  will/would drop , but the chargcontrollers destination is to keep 29 volt for say 4 hours , so himself as the gate master  ;D  will open the gate as often as needed to let the first pond overflow , but take care that the upper battery/pond stays charged  and he (a Solid State Relay) can  do that as often as needed  10 times per hour (depending how much flow into the first battery ) 50 times per hour or each second  if needed , just to keep the voltage level in the first battery at 29 V  , so ingeneral i guess its just the surplus watts  of the 2000 watt inflow that is passed through to the second battery
So there will not be an enormous dam  brake between the two ponds to overcharge

Sure the second pond needs a dump , but  there are so many things one can do  cheaply with AC voltage  combined with a cheap inverter that can start dumping via a relay at a certain voltage  , instead of looking around for expensive DC gear   .... and the main Battery/pond  stays in good performance

Where is the Whisky now   and the hat

Billi


Title: Re: Battery as a dump load
Post by: EccentricAnomaly on January 16, 2011, 08:06:49 PM
Billi would need a fair few of them to use as a dump load for his 1600watt turbine.

Like 750 worth for the second hand laptop packs for just one hours worth of dumping or 1.6kWh.

Billi's a bright chap, though.  He's not likely to let his main bank fill completely then try to divert everything to the second bank (at least if he hasn't had too many pints).

OK, I know the following numbers are highly biased but they do show that the costs of alternatives to lead-acid aren't completely out of the ball-park.

Outasight's NiCd packs: 12 V 4.5 Ah = 54 Wh.  25 so about 0.47/Wh.

Rolls Solar 4000 12 V 106 Ah = 1272 Wh.  But you only want to discharge them to, say, 20% DOD so 255 Wh usable capacity.  133.05 from Wind and Sun (http://www.windandsun.co.uk/Prices/prices_batteries.htm) so 0.52/Wh.


Title: Re: Battery as a dump load
Post by: Justme on January 16, 2011, 09:10:50 PM
But you are comparing new prices with second hand prices, not exactly cricket.


Title: Re: Battery as a dump load
Post by: biff on January 16, 2011, 09:47:48 PM
hi billi,
      have you a little still going down there ?. i hear they brew exellent moonshine down there in kerry,problem is,,,it can overexcite the dump load after a hefty charge and leave you shouting for your brown trousers,,, sh*tfan: :hysteria :hysteria
                              biff


Title: Re: Battery as a dump load
Post by: billi on January 16, 2011, 10:02:41 PM
 ;D   problem is all the gear is AC and the moonshine is not enough for the pv  to produce   some  high percentage  juice


Title: Re: Battery as a dump load
Post by: EccentricAnomaly on January 16, 2011, 11:26:19 PM
...not exactly cricket.

Indeed, hence "highly biased".

New LiFePO3 (https://www.navitron.org.uk/forum/index.php/topic,8368.msg140732.html#msg140732).  $1.35/Ah.  Probably 1.35/Ah after shipping, import duties, VAT, etc.  At 3.2 V that's 0.42/Wh.

Lead-acid only looks so cheap because the actual usable capacity is usually grossly overstated.  Perhaps somebody ought to call trading standards.


Title: Re: Battery as a dump load
Post by: Outtasight on January 16, 2011, 11:43:03 PM
But you are comparing new prices with second hand prices, not exactly cricket.

Believe it or not, the 25 NiMH packs are new.  I bought a new one for the old laptop.  It was actually way better than the original Toshiba spec. pack which was only 2.6Ah.  I remembered the capacity a bit wrongly; just checked and it's only 4.0Ah.  

They've gone up a bit in price since last year but you can still get them for 30.85.

http://www.laptopbatterypack.co.uk/cheap.php/Laptop-Battery/TOSHIBA/PA2429U.html (http://www.laptopbatterypack.co.uk/cheap.php/Laptop-Battery/TOSHIBA/PA2429U.html)

Just checked Rapid Electronics and they're doing Ansmann 10Ah NiMH D cells for 20.34 for twin packs (101.70 for a 10Ah 12V pack), but the laptop pack is cheaper per 12V Ah (7.71 against 10.17).  And the laptop packs come in handy rectangular bricks that are pre-wired with thermistors and exposed gold pads on the edge.  You could easily solder wires on to the pads directly to make up series / parallel packs.  You don't need any clever cell balancing gubbins either because NiMH is intrinsically safer than Lithium.

I'm digressing a bit from the original question here though.  I'm not suggesting that such small packs would be useful as a dump load but just that you could use them as a secondary low power store of solar power that you can run small inverter loads from to spare cranking up the big beastie.  

A certain oil company owns the patent on NiMH cells, which is why there are no large format NiMH cells produced (Toyota and Panasonic were forced to close down the production line they'd built) but the patent expires in 2015, so we'll be free to have them again.  There's a good video on YouTube about a guy with one of the few full EV Toyota RAV4 cars that were sold before Toyota were ordered to shut down their battery factory for infringing the patent.  Laptop makers and AA cell makers were exempted from the patent and Toyota had to resort to building a hybrid (the Prius) because the patent only allowed them to make small NiMH cells that couldn't be used for an EV.  

This car was on sale (well, leased) in 1997.  They had a mini SUV with a 95Ah 288V (24x 12V) pack that could do 80-120 miles per charge... It didn't need a cray supercomputer to keep the cells in balance (it had passive balancing for goodness sake!).  The oil companies didn't like it, so they bought the patent on the NiMH technology and killed it.  Practically criminal.

If you extrapolated that the Toshiba laptop pack has increased in capacity for the same size by 1.538x in the same time frame, then the Toyota EV today would have a 146Ah pack in the same size and be able to drive at least 123-184 miles today.


Title: Re: Battery as a dump load
Post by: billi on January 20, 2011, 01:07:39 PM
A general word to battery selection in my opinion is  , while battery technology improves perhaps  but in parallel prices for PV drop a lot

So why should one buy a much more expensive battery  and dump his money there , instead of saving part of that cash to buy more PV ?

If whe look again at the prices of the LiFePO3   and  take
Quote
$1.35/Ah
as a price guide  for a 3.2 volt battery  = 1080 Dollar for a 200 ah 12 volt pack  say   or  5000 dollar  for a 1000 ah  (import delivery and Vat  i guess 5000 Euro  then )

That is then about 3300 Euro more compared  to an of the shelf  new forklift battery  in that size

3300 euro buy me a 2000 watt PV then for free  ;D 
With PV prices down  so much i would rather oversize the spendings on PV than on battery

Billi



Title: Re: Battery as a dump load
Post by: EccentricAnomaly on January 20, 2011, 01:39:40 PM
That is then about 3300 Euro more compared  to an of the shelf  new forklift battery  in that size

Yes, but you can only actually use about 200 Ah of your 1000 Ah forklift battery.  You need to compare the price of 200 Ah of LiFePO4 vs the price of 1000 Ah of forklift batteries.

Also, when you have a run of dark days you don't have to sit and fret about your LiFePO4 batteries sulphating or worry, until the run gets very long, about starting up your nice reliable (?) generator.


Title: Re: Battery as a dump load
Post by: Justme on January 20, 2011, 01:56:34 PM


Yes, but you can only actually use about 200 Ah of your 1000 Ah forklift battery. 

Rubbish.

Even going on the more normal 50% rule for normal sized deep cycle bats you are way out & for forklift cells you can use 80% without too much loss of life. We keep to 50% SOC/DOD on ours. Most forklift drivers will drive them till they stop  due to system shut down on low volts.

You do have a point about the sulphating. But the 3K saving buys a lot of fuel for the genny at 50p per litre.


Title: Re: Battery as a dump load
Post by: billi on January 20, 2011, 02:02:15 PM
LiFePO4 batteries are then charged by the mighty hand of god i suppose  overnight


Title: Re: Battery as a dump load
Post by: clivejo on January 20, 2011, 04:29:07 PM
LiFePO4 batteries are then charged by the mighty hand of god i suppose  overnight


I want some of these batteries!


Title: Re: Battery as a dump load
Post by: DaveSnafu on January 20, 2011, 07:47:12 PM
Theres another factor that needs to be considered, 1200 will buy you a new 1100ah forklift battery, it,ll weigh somewhere near 500kg, it,ll last for many years, and batterys are 400 a ton at the minute, so in 5-6 years you will get 200 back in scrap, lead is not gonna go down in price.
Our batterys were second hand forklift batteries 375, we bought them 5 years ago, yes they are bumblebeed now but whatever you put in they give you back, we have run them down to 22v many times, these days in frost they are usually flat in a couple of days.
Time for another set, trouble is we can,t afford new ones, and all the ones on ebay that I have seen have all been through the same hard winter as us.
Its a toughie, but the batteries are working for us not the other way round, they won,t last for ever no matter what you do, if they last 5-6 years from new its just 200-250 a year.