Yesterday I checked our off grid battery bank and it had dropped quite low, under 11 volts. So I cranked up the generator. I came back 3 minutes later to see on the meter the voltage was already up to 12.3. Why does the voltage jump up so quickly when charging with a generator? Seriously....3 minutes is all it took.
Why does voltage shoot up so quickly when charging?
- Thread starter spendlove
- Start date
gnubie
Solar Wizard
- Joined
- Sep 20, 2019
- Messages
- 3,847
11V was measured under load / excessive charging current / batteries are degraded.
Pick your poison.
Measuring voltage under load does not give a true indication of the state of the battery. When charging is performed the load will be largely powered by the charger so the battery floats up to its resting voltage and of course is dragged higher by the act of charging.
Excessive charging current will cause the voltage to rise faster than would normally be seen simply because the battery can't cope with the current available, ie chemical reactions can't take place fast enough etc.
If your batteries are aged, sulphated, dry, or any other number of possibilities they will have considerably reduced capacity and will rise up in voltage without taking on much 'charge'.
Pick your poison.
Measuring voltage under load does not give a true indication of the state of the battery. When charging is performed the load will be largely powered by the charger so the battery floats up to its resting voltage and of course is dragged higher by the act of charging.
Excessive charging current will cause the voltage to rise faster than would normally be seen simply because the battery can't cope with the current available, ie chemical reactions can't take place fast enough etc.
If your batteries are aged, sulphated, dry, or any other number of possibilities they will have considerably reduced capacity and will rise up in voltage without taking on much 'charge'.
What size is your battery bank? What amperage is your generator charging at? What chemistry is the battery? How old are they?
Sverige
A Brit in Sweden
Because batteries have an internal resistance, which causes a voltage rise across the battery terminals in response to charging churrent flowing into the battery. The same internal resistance causes a voltage drop across the terminals when current flows out of the battery into a load.
So although your voltage rose quickly in response to the charging from the genny, you would have seen it drop back down quick enough if you shut it off, and only then does the battery voltage give you a true reflection of the extra charge you’ve put into the battery.
The problem I am seeing now, is that there is not enough sun to charge my batteries and use them at the same time, and I am finding I am in the workshop more than I anticipated this winter (blame covid). I have a meter that measures energy in and out not voltage referenced, and I also have an inverter that will flip between mains and battery as per voltage settings, and also contains an 85A battery charger.
So lets say I discharge to 50% (according to the meter), but the voltage is still high because there are only moderate loads - the charger very quickly goes to a float status, even though there are 500AH missing. At float it would take all week to put back in - and these batteries are only a few months old. I simply can't charge the batteries as they wont accept the current.
So following this thread are you saying I need to reduce the charge current to a more acceptable level to get them to accept it.
these are brand new AGM batteries, 1040AH but only ever take 500AH out of them before I ether mains charge or shut shop for them to recover.
It seems the charger sees the voltage and thinks they are full.
So lets say I discharge to 50% (according to the meter), but the voltage is still high because there are only moderate loads - the charger very quickly goes to a float status, even though there are 500AH missing. At float it would take all week to put back in - and these batteries are only a few months old. I simply can't charge the batteries as they wont accept the current.
So following this thread are you saying I need to reduce the charge current to a more acceptable level to get them to accept it.
these are brand new AGM batteries, 1040AH but only ever take 500AH out of them before I ether mains charge or shut shop for them to recover.
It seems the charger sees the voltage and thinks they are full.
gnubie
Solar Wizard
- Joined
- Sep 20, 2019
- Messages
- 3,847
What voltage is your charger changing to float mode? Absorption for a typical AGM is 14.2 to 14.7V. Once there the charger should not just switch back, it should hold there until the current tails off, for a 1040AH battery call it 30A, or until many hours have passed assuming it has a time limit.
Lifting a half charged 1040AH battery to full charge is going to take many hours with an 85A charger. You'll get to 60% SOC in 2-3 hours or so, from then on it's a long march to full capacity, another 5 hours or perhaps more. This assumes your battery is in good condition, and you took 500AH out of it.
Lifting a half charged 1040AH battery to full charge is going to take many hours with an 85A charger. You'll get to 60% SOC in 2-3 hours or so, from then on it's a long march to full capacity, another 5 hours or perhaps more. This assumes your battery is in good condition, and you took 500AH out of it.
I bet that's it then, because it starts at 75A and then drops to about 3 amps after a few hours, even though the battery is hungry for more. Its a 12v system, not the best I know, but when this inverter packs up I will go with higher voltage. It floats at 13.8 and boosts at 14.4 and I disabled equalise. Its all configurable in the menu (within reason).
gnubie
Solar Wizard
- Joined
- Sep 20, 2019
- Messages
- 3,847
Monitor your battery voltage and current directly, ie use a decent meter. A clamp multimeter etc. Ensure your charger is actually going to absorption/boost voltage. Monitor the amps flowing. If they rapidly fall once you reach absorption voltage either your battery was not really half discharged, or it's crook. The values in this graph do not refer to your specific battery & charger, but they give you an idea of what to expect in terms of percentages.
svetz
Works in theory! Practice? That's something else
If you did stop the charging cycle before measuring, you might not have waited long enough.
There's a "plate effect" (e.g., lots of electrons on the plate that haven't made it into ions yet), typically you have to let them sit 15 minutes or so after charging to get the actual voltage/state-of-charge.
There's a "plate effect" (e.g., lots of electrons on the plate that haven't made it into ions yet), typically you have to let them sit 15 minutes or so after charging to get the actual voltage/state-of-charge.
RCinFLA
Solar Wizard
- Joined
- Jun 21, 2020
- Messages
- 3,560
All rechargable batteries have two components changing its terminal voltage. First is effective internal resistance of internal connections and electrolyte resistance. Second is kinetic energy to create and move ions.
Lead acid is fairly high on both (compared to LFP). Lead-acid battery has to breakup and recombine a lot of different molecules. This take more overhead energy which manifests itself as added terminal voltage drop during discharge and terminal voltage rise during charging. When battery gets near full charge or near full discharge it is harder to find available molecules needed to convert to meet current demand and kinetic voltage goes up.
There is a third component effecting rechargable batteries during charging. It is called surface charge which is effectively an electrolytic capacitor charge that occurs at layer surfaces. Different chemistry batteries have different amount of surface charge amount and its dissipation rate. Lead acid is moderate, LFP is very low columbic charge amount and dissipates quickly.
Lead acid is fairly high on both (compared to LFP). Lead-acid battery has to breakup and recombine a lot of different molecules. This take more overhead energy which manifests itself as added terminal voltage drop during discharge and terminal voltage rise during charging. When battery gets near full charge or near full discharge it is harder to find available molecules needed to convert to meet current demand and kinetic voltage goes up.
There is a third component effecting rechargable batteries during charging. It is called surface charge which is effectively an electrolytic capacitor charge that occurs at layer surfaces. Different chemistry batteries have different amount of surface charge amount and its dissipation rate. Lead acid is moderate, LFP is very low columbic charge amount and dissipates quickly.
If the battery is discharged to 50% (or about 500AH) then boost charging at 75A should last about 5 hours. An indicator of high cable resistance is if charging current drops to ~3A after a few hours . Charging cable should be at least 1/0. Also, 14.4V boost is quite low.
What brand converter are you using? Which batteries?
Hi guys I have sorted it. The problem is that if I turn off and on the charger/inverter whilst charging at high current, it will then think the batteries are fully charged, and go in to float mode. I must have done this whilst working on the system, and never made the connection.
The batteries and inverter are these.
The wire I use is this
The batteries and inverter are these.
Leoch Adventurer AGM 12V 130Ah Deep Cycle Solar, Leisure Marine Battery LAGM-130 | eBay
Find many great new & used options and get the best deals for Leoch Adventurer AGM 12V 130Ah Deep Cycle Solar, Leisure Marine Battery LAGM-130 at the best online prices at eBay! Free delivery for many products.
www.ebay.co.uk
Mophorn 3000W 12V Low Frequency Pure Sine Wave Off-Grid Power Inverter 230V 3Kw Nominal 9Kw Peak Power AC Charger: Amazon.co.uk: Business, Industry & Science
Mophorn 3000W 12V Low Frequency Pure Sine Wave Off-Grid Power Inverter 230V 3Kw Nominal 9Kw Peak Power AC Charger: Amazon.co.uk: Business, Industry & Science
www.amazon.co.uk
The wire I use is this
4 GAUGE OFC RED 25mm² POWER CABLE 4 AWG OXYGEN FREE COPPER PER METRE | eBay
Find many great new & used options and get the best deals for 4 GAUGE OFC RED 25mm² POWER CABLE 4 AWG OXYGEN FREE COPPER PER METRE at the best online prices at eBay! Free delivery for many products.
www.ebay.co.uk
4 awg cable may or may not be adequate. You want to design to a 1% voltage drop, or cable voltage drop of 0.144V.
At 0.21 m Ohm/ft cable resistance and 75 A charging current, the maximum cable length is:
cable length = 0.144 V / (0.21 mOhm/ft * 75A) = 9 ft (round trip)
The charging source can be max 4.5 ft from battery to maintain 1% voltage drop.
At 0.21 m Ohm/ft cable resistance and 75 A charging current, the maximum cable length is:
cable length = 0.144 V / (0.21 mOhm/ft * 75A) = 9 ft (round trip)
The charging source can be max 4.5 ft from battery to maintain 1% voltage drop.
Similar threads
