Problems with DIY battery system for p.m. computer use

[two sheds]

Anyone help? Hope I’m ok posting this with embedded links.

I have solar panels but no battery backup, and I wanted to power my computer of an evening with my day’s excess power. I bought this charger, these two 12V lead acid batteries I’m connecting in parallel, and this 3 kW inverter.

I was hoping for more than 8 hours supplying my just under 100 W computer from the 260 Ah (3.1 kWh) batteries. A month ago when I started, the charger was supplying 0.69 kWh into the two parallel batteries, and the inverter was supplying 0.47 kWh into the computer, translating to about 5 hours. Overall efficiency was around 68% which I expected with the batteries not being hugely efficient because they’re discharging within a few hours, and 90% efficiencies for charger and inverter.

The power supplied by the inverter has dropped steadily until it’s now only 0.28 kWh with efficiency of 63% so just under 3 hours computer use. I’m so far assuming it’s not the (new) batteries but don’t know whether it’s the charger or the inverter that’s the problem. I presume one of them is estimating the wrong amount of charge left in the batteries. One place in the charger instructions (and on their website) says they suit up to 200 Ah but another place says 120 Ah.

Is it assuming that the battery is discharged when it’s supplied the 200 Ah? The inverter also seems to think that the battery needs recharging (shown by lights on the control panel) when there should be a fair amount left in.

Any thoughts, people? And can I solve it by for example putting a switch like this in front of the 12 V positive battery terminals and switch from one to other half way through the charging?

Thanks

 

[sunshine_eggo]

Battery specs:

Capacity (C5)​	80 Ah​
Capacity (C20)​	100 Ah​
Capacity (C100)​	130 Ah​

C5 = 16A - 16A current will last 5 hours
C20 = 5A - 5A current will last 20 hours
C100 = 1.3A - 1.3A current will last 100 hours

Based on the above, your battery has a Peukert exponent of 1.19

100W/12V = 8.3A meaning your going you have something between 80 and 100Ah of capacity, not 130Ah.

At 8.3A, your battery capacity is 91Ah, so that's about 11 hours of run time until a single battery is completely dead. Obviously, you don't want to do that, so with two batteries in parallel, 11 hours of run time should discharge each to about 50%.

This does not take into consideration efficiency or self consumption.

Charger indicates a maximum charge voltage of 14.2V, and it doesn't indicate how long it will dwell at that voltage.

It's really hard to glean what's going on, but assuming 14.2V, you need to charge for at least 6.5 hours per day @ 10A to replenish the 100W you plan to use for 8 hours - longer if charging at less that 14.2V. Note that you have to replace about 115% of the capacity used due to lead-acid charging inefficiencies.

If 14.2V is below the recommended absorption voltage for your battery, it may never fully charge it.

Just found the datasheet:

https://cdn.tayna.com/datasheets/solarflex-SFL130MF-datasheet-2.pdf

I don't recall seeing Peukert published, but there it is... 1.2. Got close enough.

14.4V charging voltage.

I suspect you are simply not allowing enough charge time, have poor connection to the battery (clamps will make the charger see a higher voltage than the battery is at - you should be using the ring terminals pictured in the listing), and the charger is not charging to 14.4V

Before you continue, I would allow the charger to charge the battery bank to full. Insure you see them at 14.2V (measured with a voltmeter AT the battery terminals) for at least a couple hours.

 

[Substrate]

You connected them in parallel. Did you charge each one up *individually* at first before putting them in parallel to operate and charge?

Extremely important to do this initial individual charge with leisure batts. Helps ensure that each internal cell is fully charged - balanced - and this is easier to accomplish with only 6 cells in series on an individually charged batt, than it is to do in parallel. It's called a "commissioning" charge.

Note too that most simple chargers, to protect the consumer from trying to charge bad batteries, will "drop to float" too soon. They rely on having at least 8 or more hours of float to actually finish the job - and that's AFTER you may have gotten any sort of full-charge light!. They aren't.

Or, worse still, they may drop to a "maintenance float", really low in voltage, and only trigger back on when they have self-discharged to a puny 13.4v or lower. I'd have to see the specs for that charger to see what they are doing.

 

[two sheds]

Appreciated @sunshine_eggo , thank you . I'd read about the 80 Ah actual rather than 130 Ah but hadn't really understood what that would mean so thanks for that, too.

When I checked early on the charger was putting out
* 120 W so about 10 A for 2.5 hours with a voltage of 12.9 V, then
* 108 W for another 3 hours at 13.2 V,
* 90 W for half an hour at 13.8 V then
* 40 W for another hour at 14 V then
* 4 W at 14.2 when it showed battery 'full'.

Those voltages are as measured by the charger rather than experienced by the battery so I'll take a look at the setup tomorrow and get back to you.

 

[two sheds]

You connected them in parallel. Did you charge each one up *individually* at first before putting them in parallel to operate and charge?

Extremely important to do this initial individual charge with leisure batts.

No I didn't ? Next stage is clearly to do that.

Note too that most simple chargers, to protect the consumer from trying to charge bad batteries, will "drop to float" too soon. They rely on having at least 8 or more hours of float to actually finish the job - and that's AFTER you may have gotten any sort of full-charge light!. They aren't.

Or, worse still, they may drop to a "maintenance float", really low in voltage, and only trigger back on when they have self-discharged to a puny 13.4v or lower. I'd have to see the specs for that charger to see what they are doing.

As above the charger is showing 14.2 V at the end, when it's putting out 4 W (all the wattages measured by a power meter I plug into the wall socket). Should I leave it charging for a few hours longer?

 

[Substrate]

I'd have to see the specs, but many hours of float is always good.

Know that automotive type chargers don't necessarily have non-automotive use in mind and may not really try to attempt to fully charge.

They may be assuming that after you use this, you will be putting the battery back into a vehicle, give it a start, and the vehicle itself will finish the charge!

But yeah, give the individual charge to each a go at first. Super important with leisure batt agm's.

 

[Substrate]

Small point - I noticed these are "MF" batteries, which means they are flooded, but "maintenance free". What that really means is that these flooded batts have extra electrolyte in them so that a consumer can ignore checking the electrolyte level for about 4 years before they replace them.

You on the other hand, may be willing to check these levels and top off with distilled water if need be, or if the caps aren't totally glued shut. (different from sealed agm, where you don't do this under any circumstance)

Also make sure that if these have "accessory posts", do NOT use those for your main interconnects, as tempting as that might be, with the commonly available little jumpers.

 

[two sheds]

Hello – me again. Apologies for the delayed response and thanks for responses last time. Some answers to your remarks/questions: first, answering a couple of your questions:

I’m using ring terminals but there are two connections to each terminal so one of the connections is clamped to the ring terminal.

I charged the batteries separately but it doesn’t seem to make any difference. I indeed didn’t realize they needed distilled water after a few years, so thanks for that.

I’ve taken another look and spoken to the people I got the charger from. They’ve said they’ll test it for me. I’m thinking of sending the information below to them with the charger. Does it suggest where the problem lies? (I was told to press the Mode button for recharging batteries that are in poor condition but that doesn’t help. They also say they use the same batteries without problems).

---

I was expecting the charger to give around 10 A for 6 hours (720 Wh) into the two 80 Ah batteries to run my 90 W computer for 8 hours for the evening (theoretical maximum would I think be 10 A for up to 16 hours, enough to supply the computer for a good 20 hours).

Instead it initially supplied around:
2.5 hours @ 10 A ,
3 hours @ 9 A,
0.5 hours @ 8 A,
3 hours @ 3 A,
then trickle charge @ 0.3 A.
(total 65 Ah so 780 Wh, with a total charger/battery/inverter efficiency of around 60% only giving me around 470 Wh so 5 hours for the computer).

I got that the first day I charged the batteries, but each following day I get between 5 and 10% less, now down to around 220 Wh so only 2.4 hours. I’d be surprised if it’s the batteries since I’ve charged each separately and got similar results. They’re fairly new, too, actually only a couple of months older than the charger.

When I press the Mode button the charger shows that it goes up to 15 V but down again to 13.1 V within about a minute and then reads full. Would this happen if the batteries were poor? The inverter at the beginning of the evening reads 12.5 V, down to 11.2 V when it shuts down.

Over the last couple of days I’ve trickle charged for 35 hours and the voltage at the charger terminals reads 13.7 V and at the battery terminals 13.3 V. I turned on the inverter and it measured the battery voltage as varying between 12.9 V to 13.3 V.

---

Thanks

 

[mikefitz]

Its not clear where solar is involved but I guess the house has solar panels and you are charging the batteries from the house 230 AC supply ?

Batteries.

The batteries you have are low cost automotive starter batteries branded as leisure deep cycle batteries. Whilst they may well serve a RV used infrequently for low power loads, they are not true deep cycle batteries and will quickly loose capacity if cycled to a low state of charge. Its possible your batteries now have a much reduced capacity. In the UK its not posible to purchace a true deep cycle lead acid battery at less than £200 per 100Ah, expect to pay between £200 and £400, example,

27TMX Trojan Battery Deep Cycle

27TMX Trojan 12V Deep Cycle Flooded Battery Trojan Batteries was founded in 1925 and has become one of the world's leading manufacturer of deep-cycle, Solar and motive batteries. Trojan produces a broad range of energy storage solutions including deep-cycle Flooded, AGM, Gel and Lithium...

www.tayna.co.uk

From your reports it suggests the inverter is terminating at low battery volts. Discharging the batteries you have to this extent will quickly reduce capacity. Sterling Power tested a number of batteries to a deep discharged state, the standard lead acid battery lost significant capacity a few a few discharge cycles.


The number of useful cycles depends on the depth of discharge, chart Trojan industrial range,


Battery Charger.

Its possible the battery charger you are using is not charging the batteries correctly. This automotive type is intended to maintain a car battery of around 80 Ah connected for long periods. A better charger for cyclic charging with programamable charge profile would be an improvement,

Victron Blue Smart IP22 Bluetooth Battery Charger - 12V 20A, 1 output

Victron Blue Smart IP22 6-stage battery charger 12V 20A (1 output). Multi-stage, Bluetooth-enabled charger for boats, caravans, motorhomes, campervans.

www.12voltplanet.co.uk

Battery Monotoring.

Being able to monitor battery status is useful in battery management.

Victron SmartShunt 500A

Fully-featured battery monitor that connects by Bluetooth to your smartphone to display a range of parameters inc. Volts, Amps, %charge, Ah consumed & more.

www.12voltplanet.co.uk

Inverter.

A 3000 watt inverter is not very efficient to power a 100 watt computer. Either a lower power inverter or a 12v computer PSU would be more ideal.

12V DC Input PC ATX Power Supplies, 12 volt Input, 1000 watts, including 83Amps at 12V regulated output

1000W 12VDC DC Input ATX Computer Power Supply, tower power supply with 12VDC input, useful in solar system, heavy equipment, boats.

www.powerstream.com

Cables.

You may have issues with voltage drops that could be improved with more ideal cable and connections. The report of a 0.4 volt drop between charger and battery under float charge is a worry. This could impact charging and inverter shutdown if volt drops are significant.

Mike

 

[two sheds]

Its not clear where solar is involved but I guess the house has solar panels and you are charging the batteries from the house 230 AC supply ?

Batteries.

The batteries you have are low cost automotive starter batteries branded as leisure deep cycle batteries. Whilst they may well serve a RV used infrequently for low power loads, they are not true deep cycle batteries and will quickly loose capacity if cycled to a low state of charge. Its possible your batteries now have a much reduced capacity. In the UK its not posible to purchace a true deep cycle lead acid battery at less than £200 per 100Ah, expect to pay between £200 and £400, example,

27TMX Trojan Battery Deep Cycle

27TMX Trojan 12V Deep Cycle Flooded Battery Trojan Batteries was founded in 1925 and has become one of the world's leading manufacturer of deep-cycle, Solar and motive batteries. Trojan produces a broad range of energy storage solutions including deep-cycle Flooded, AGM, Gel and Lithium...

www.tayna.co.uk

From your reports it suggests the inverter is terminating at low battery volts. Discharging the batteries you have to this extent will quickly reduce capacity. Sterling Power tested a number of batteries to a deep discharged state, the standard lead acid battery lost significant capacity a few a few discharge cycles.
View attachment 137703

The number of useful cycles depends on the depth of discharge, chart Trojan industrial range,
View attachment 137704

Battery Charger.

Its possible the battery charger you are using is not charging the batteries correctly. This automotive type is intended to maintain a car battery of around 80 Ah connected for long periods. A better charger for cyclic charging with programamable charge profile would be an improvement,

Victron Blue Smart IP22 Bluetooth Battery Charger - 12V 20A, 1 output

Victron Blue Smart IP22 6-stage battery charger 12V 20A (1 output). Multi-stage, Bluetooth-enabled charger for boats, caravans, motorhomes, campervans.

www.12voltplanet.co.uk

Battery Monotoring.

Being able to monitor battery status is useful in battery management.

Victron SmartShunt 500A

Fully-featured battery monitor that connects by Bluetooth to your smartphone to display a range of parameters inc. Volts, Amps, %charge, Ah consumed & more.

www.12voltplanet.co.uk

Inverter.

A 3000 watt inverter is not very efficient to power a 100 watt computer. Either a lower power inverter or a 12v computer PSU would be more ideal.

12V DC Input PC ATX Power Supplies, 12 volt Input, 1000 watts, including 83Amps at 12V regulated output

1000W 12VDC DC Input ATX Computer Power Supply, tower power supply with 12VDC input, useful in solar system, heavy equipment, boats.

www.powerstream.com

Cables.

You may have issues with voltage drops that could be improved with more ideal cable and connections. The report of a 0.4 volt drop between charger and battery under float charge is a worry. This could impact charging and inverter shutdown if volt drops are significant.

Mike

That's looks superb information, thank you.

At first sight it looks like I have the wrong charger charging the wrong batteries connected to the wrong inverter using the wrong cables. :D

All good to know, and I'll go off and study it and come back to you.

 

[two sheds]

Its not clear where solar is involved but I guess the house has solar panels and you are charging the batteries from the house 230 AC supply ?

Batteries.

The batteries you have are low cost automotive starter batteries branded as leisure deep cycle batteries. Whilst they may well serve a RV used infrequently for low power loads, they are not true deep cycle batteries and will quickly loose capacity if cycled to a low state of charge. Its possible your batteries now have a much reduced capacity. In the UK its not posible to purchace a true deep cycle lead acid battery at less than £200 per 100Ah, expect to pay between £200 and £400, example,

27TMX Trojan Battery Deep Cycle

27TMX Trojan 12V Deep Cycle Flooded Battery Trojan Batteries was founded in 1925 and has become one of the world's leading manufacturer of deep-cycle, Solar and motive batteries. Trojan produces a broad range of energy storage solutions including deep-cycle Flooded, AGM, Gel and Lithium...

www.tayna.co.uk

From your reports it suggests the inverter is terminating at low battery volts. Discharging the batteries you have to this extent will quickly reduce capacity. Sterling Power tested a number of batteries to a deep discharged state, the standard lead acid battery lost significant capacity a few a few discharge cycles.
View attachment 137703

The number of useful cycles depends on the depth of discharge, chart Trojan industrial range,
View attachment 137704

Battery Charger.

Its possible the battery charger you are using is not charging the batteries correctly. This automotive type is intended to maintain a car battery of around 80 Ah connected for long periods. A better charger for cyclic charging with programamable charge profile would be an improvement,

Victron Blue Smart IP22 Bluetooth Battery Charger - 12V 20A, 1 output

Victron Blue Smart IP22 6-stage battery charger 12V 20A (1 output). Multi-stage, Bluetooth-enabled charger for boats, caravans, motorhomes, campervans.

www.12voltplanet.co.uk

Battery Monotoring.

Being able to monitor battery status is useful in battery management.

Victron SmartShunt 500A

Fully-featured battery monitor that connects by Bluetooth to your smartphone to display a range of parameters inc. Volts, Amps, %charge, Ah consumed & more.

www.12voltplanet.co.uk

Inverter.

A 3000 watt inverter is not very efficient to power a 100 watt computer. Either a lower power inverter or a 12v computer PSU would be more ideal.

12V DC Input PC ATX Power Supplies, 12 volt Input, 1000 watts, including 83Amps at 12V regulated output

1000W 12VDC DC Input ATX Computer Power Supply, tower power supply with 12VDC input, useful in solar system, heavy equipment, boats.

www.powerstream.com

Cables.

You may have issues with voltage drops that could be improved with more ideal cable and connections. The report of a 0.4 volt drop between charger and battery under float charge is a worry. This could impact charging and inverter shutdown if volt drops are significant.

Mike

Thanks again for taking the time to give such an expert reply. Yes, I have solar panels but no battery backup. I was hoping to charge the batteries during the day with the surplus energy that I would otherwise just be paid 4p/kWh for. The system cost a couple of hundred pounds and with near 1 kWh/night (8 hours of computer use) at a possible future 50p/unit I was hoping for a repayment time of about 2 years.

I’d thought my first reading was a mistake but the first evening I measured 700 Wh for the two batteries so 350 Wh each which is still only 30 Ah rather than the 80 Ah I was expecting. That dropped over two months to 200 Wh = 16 Ah per battery. I’ve just had the batteries on charge for 14 days so a trickle charge of around 400 Wh from the panels. That gave slightly more computer time at 3 hours.

You say repeated charging at low voltage will have damaged the batteries. Should not the charger at first at least have charged to higher voltage? Pressing the “mode” button is supposed to do this for batteries with low charge, but it does nothing. Is it worth having the manufacturers check it, although even they found it was faulty I’m not sure that would help me much. Similar with the battery manufacturers to be honest.

The system you suggest indeed looks much more suitable but I’d be stretching the repayment time (and assuming no drop in capacity over a couple of years). And a Lithium-ion battery would never repay (I can't actually add battery backup to my existing solar system).

I’m now wondering what to do next. I’ve no real use for them in one of my sheds (and they’d likely freeze in a bad winter), so I’m thinking of just keeping them charged up for say an hour a day from the solar panels and use them purely in emergencies. I’ve got a UPS for my main computer upstairs but that is just to let me shut it down if there’s a power cut (I’m out in the countryside so that does happen). Having one downstairs for my backup computer would give me a couple of hours for the computer and phone during a power cut, and I could live with that.

So, thanks again everyone who replied.