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Old 21st December 2015, 02:17 PM   #1
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Default Charging/maintaining 18v of SLA battery with 20v laptop PSU?

I'm about to set up an 18v SLA supply for my 2 SMSL SA36A Pros using new 12V and 6V batteries (both will be new 12AH Yuasas) in series.

(Why not 3 x 6V? tidier and simpler, the Yuasa's are the same dimensions except depth).

I have a largish (6A) 20V laptop power supply which I've been using to power the SMSL's. it's very tightly regulated - 20.1V open and almost no drop under load, which is to say with the 2 SMSL's playing bass-heavy music at or near flat-out, (2-3A RMS?).

Yes, the SMSL's sound fine with the brick, but having had nice results using a 12V SLA with both them and various Tripaths, I'd like to try them with 18V (actually nearer 19V) battery power.

So:- coincidentally, 20V happens to be as near as dammit exactly the float for 18V of SLA at the temperatures they'd likely be operating at.

Assuming I was *careful* and didn't let them discharge to less than c.50% (i.e. keep them over 18V), which would easily allow an evening (maybe an entire day) of listening at quite high volume, would the laptop brick effectively 'trickle-charge' and then float-charge them if I left it on overnight?

I've been trying to figure out how much current the batteries would draw from the PSU with that 2V differential.

IOW, could the batteries draw too much current for their own or the PSU's good?

Conversely, could they draw too little from 20V to charge overnight from c. 50%?

The reasoning for all this is that it adds up to a simple and **cheap!** topology. It would save having to buy smart/maintenance chargers which would otherwise be the most expensive components.

Even if I didn't already have the brick, good ones can be had for 10-20. Nothing more needed than a few spade connectors, a couple of inline fuses and switches (which I already have) and I would never be without power for the amplifiers (well, as long the grid's still up).

I'm going to try it anyway and if it *seems* to work, initially taking frequent voltage readings (and maybe even current readings), but I'm interested in hearing in advance from anyone more experienced/knowledgeable as to whether it's feasible (I won't have the batteries until into the new year) .

Last edited by Aleksunder; 21st December 2015 at 02:23 PM.
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Old 21st December 2015, 02:30 PM   #2
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A fully charged 12 volt cell measures 13.6 volts and a 6 volt cell 6.8 volts so that is about right. They will almost receive a full charge.
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Old 21st December 2015, 02:42 PM   #3
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The risk with series connecting is that charge inequality will result in some cells in the chain seeing to much voltage as they reach a fully charged state. This will also reduce the current available to the other less charged cells.

Not a good idea imo.

As to charge currents, a discharged or partially discharged battery in good shape (with low internal resistance) will draw pretty much whatever the charger can supply, with the batteries drawing up to 10's of amps if it were available. So if the charger isn't limited then you could well run into problems there too.
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Old 21st December 2015, 03:13 PM   #4
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Originally Posted by Mooly View Post
The risk with series connecting is that charge inequality will result in some cells in the chain seeing to much voltage as they reach a fully charged state. This will also reduce the current available to the other less charged cells.

Not a good idea imo.
Yes, I've considered that. But the Yuasa's will be identical in everything but number of cells, as close as you can get to a single 'true' 18V SLA battery (ETA > apart from 3 identical 6V, of course).
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Originally Posted by Mooly View Post
As to charge currents, a discharged or partially discharged battery in good shape (with low internal resistance) will draw pretty much whatever the charger can supply, with the batteries drawing up to 10's of amps if it were available. So if the charger isn't limited then you could well run into problems there too.
This is the nub. Do you have an idea of how much current an 18V/12AH battery would draw at c. 18.2V from 20V?

What I might try at first is charging from gradually diminishing battery charge/voltage and see if there are signs of distress from the brick. I assume the worst that's likely to happen is waving it bye-bye.

I guess that I'd pretty much have the PSU on all the time, acting as a float charger (which I'm pretty certain would be fine), only switching it off for 'critical' listening (assuming the amps do actually sound better with the batteries), hopefully rarely getting below c. 80%.

Last edited by Aleksunder; 21st December 2015 at 03:17 PM.
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Old 21st December 2015, 03:23 PM   #5
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As a rule, one should not exceed 10% of the rated current of the battery to charge it. So a 12A/H battery should be charged with no more than 1.2 Amps.
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Old 21st December 2015, 03:47 PM   #6
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I'm trying to apply my pretty basic understanding of electricity (V/I/R), but battery physics/chemistry complicates things beyond it;

So (JonSnell) bulk-charging of an 18V 12AH SLA would be at about 22V but current limited to 1.2A (optimally, for long battery life - I know occasional fast charging isn't disastrous).

The question is whether a lower voltage, i.e. the tightly-regulated 20V from the brick, intrinsically current-limits as long as the battery is only partly discharged?
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Old 21st December 2015, 04:48 PM   #7
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Limiting the charge current will stop the cells from boiling dry and destroying themselves. Have a look at Yuasa Batteries for correct operation.
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Old 21st December 2015, 06:03 PM   #8
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The Yuasa document you want is this one Yuasa Little Red Book of Batteries.pdf

Even reducing the charge level by a few percent on a SLA battery means that it will still draw huge current for a short time from the charger.

One possible solution would be to include a shottky diode in the feed from the battery to the amp and then parallel that diode with a low value resistor to limit the charge current when its charging. Its a compromise solution but would safeguard the charger.
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Old 21st December 2015, 07:33 PM   #9
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The Yuasa document you want is this one Attachment 520469

Even reducing the charge level by a few percent on a SLA battery means that it will still draw huge current for a short time from the charger.

One possible solution would be to include a shottky diode in the feed from the battery to the amp and then parallel that diode with a low value resistor to limit the charge current when its charging. Its a compromise solution but would safeguard the charger.

Ah - you pre-empted. I'd heard about the use of diodes. That would drop the voltage the amp sees from the battery by 0.5-0.6v?

I'm beginning to see the issues, and why so much effort goes into designing good charging systems - noticed that internal resistance in SLA's is measured in mΩ, hence even a couple of volts can lead to a LOT of current flowing.

Still, worth a try - at worst I'll be forced to proper smart chargers.

ETA >> forgot; thanks for the link, very useful indeed.

Last edited by Aleksunder; 21st December 2015 at 07:38 PM.
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Old 21st December 2015, 07:46 PM   #10
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The diode would lose a little voltage, and a shottky would be nearer 0.3 volts. If you are just worried about the loss in maximum output from the amp then it would be negligible at those voltage drops.
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