Re: [AH] Does mAh value matter on a memory backup battery?

From Peter MoĢˆrck
Sent Fri, Jan 24th 2014, 14:25

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My point about being careful when replacing NiCad with NiMH was that they
are chemically not the same and require slightly different charging
techniques.
The charge circuit was likely designed by recommendations from the NiCad
battery manufacturers.
If the engineers had been able to use NiMH cells from the start I guess the
circuit would have looked slightly different (maybe only slightly different
charge current).

It's hard to find reliable info about this, but here's one site that does
some explaining:
http://batteryuniversity.com/learn/article/charging_nickel_metal_hydride

IIRC a NiMH won't hold it's charge for as long as a NiCad does and the NiMH
should have a lower trickle charge current.
They may also need higher charge voltage per cell compared to NiCad.

http://www.ti.com/lit/an/snva557/snva557.pdf
"...most Ni-Cd cells will easily tolerate a sustained charging current of
c/10 (1/10 ofthe cell's A-hr rating) indefinitely with no damage to the
cell."
"Ni-MH cells are not as tolerant of sustained charging: the maximum safe
trickle
charge rate will be specified by the manufacturer, and will probably be
somewhere
between c/40 and c/10"

So yes, they're similar, but they're not the same and I wouldn't bet my
life on that it doesn't matter.

I replace NiCads with Litium cells and modify the circuit to prevent charge=
.
If I knew how to calculate the correct charge current for NiMH I might have
chosen to modify the circuit for that instead.

I choose to play it safe.
Your choice is yours :)

Regards,
Peter



On 24 January 2014 12:38, Steve Lenham <steve@bendentech.co.uk> wrote:

> I have never encountered a synth with a NiCd/NiMH memory backup battery
> that does not simply trickle charge it with a small current - in that
> situation, you can use NiCd and NiMH interchangeably, and I do so
> regularly. It's only if you want to rapid-charge them that it matters.
>
> The OP wrote that it is a memory backup battery, so the current drain is
> negligible and the capacity uncritical - the battery is probably
> self-discharging faster than it being drained by the CMOS RAM device that
> it maintains.
>
> So the answer to the original question is that either would be OK, but if
> the larger capacity device will physically still fit then there is no
> reason not to go for that one.
>
> Cheers,
>
> Steve L.
>
> Benden Sound Technology
> www.bendensound.co.uk
>
>
>
> On 23/01/2014 19:47, Peter M=F6rck wrote:
>
>> OK, get another NiMH then, should be no problem :)
>> Old synths usually have NiCad and the recharging circuit probably isn't
>> designed to handle NiMH (which has different charging characteristics).
>>
>>
>>
>> On 23 January 2014 20:42, M V <mv_synths@hotmail.com
>> <mailto:mv_synths@hotmail.com>> wrote:
>>
>>     Yes, it's an NIMH battery... on a pretty small synth.
>>
>>
>>
>>
>>     ------------------------------------------------------------
>> ------------
>>
>>     Date: Thu, 23 Jan 2014 20:40:24 +0100
>>     Subject: Re: [AH] Does mAh value matter on a memory backup battery?
>>     From: peter.morck@gmail.com <mailto:peter.morck@gmail.com>
>>     To: mv_synths@hotmail.com <mailto:mv_synths@hotmail.com>
>>     CC: analogue@hyperreal.org <mailto:analogue@hyperreal.org>
>>
>>
>>
>>     The big question is: Is it a rechargeable battery?
>>     100mAh sounds awfully low though, so I'm guessing no.
>>     Is it a toy? :P
>>
>>     Capacity is a measure of how much charge the battery has.
>>     100mAh will last for 100 hours if the synth drains 1mA from it, 1000
>>     hours if it drains 0.1mA from it, and so on... (in theory)
>>     So, bigger is better as it will last longer.
>>
>>     /Peter
>>
>>
>>
>>
>>
>>     On 23 January 2014 20:33, M V <mv_synths@hotmail.com
>>     <mailto:mv_synths@hotmail.com>> wrote:
>>
>>         I'm looking to replace a memory backup battery on a synth. The
>>         original was a 3.6V 100mAh battery, but the available mAh values
>>         for new ones are 80 and 150. I'm guessing that such a small
>>         difference won't matter, but am unsure whether it's better to
>>         shoot high or low.
>>
>>         thanks,
>>
>>         Matt
>>
>>
>>
>>
>>
>

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<div dir=3D"ltr"><div><div><div><div><div><div><div><div><div><div>My point=
 about being careful when replacing NiCad with NiMH was that they are chemi=
cally not the same and require slightly different charging techniques.<br>
</div>The charge circuit was likely designed by recommendations from the Ni=
Cad battery manufacturers.<br></div></div>
If the engineers had been able to use NiMH cells from the start I guess the=
 circuit would have looked slightly different (maybe only slightly differen=
t charge current).<br><br></div>It&#39;s hard to find reliable info about t=
his, but here&#39;s one site that does some explaining:<br>

<a href=3D"http://batteryuniversity.com/learn/article/charging_nickel_metal=
_hydride" target=3D"_blank">http://batteryuniversity.com/learn/article/char=
ging_nickel_metal_hydride</a><br><br></div>IIRC a NiMH won&#39;t hold it&#3=
9;s charge for as long as a NiCad does and the NiMH should have a lower tri=
ckle charge current.<br>
</div><div>They may also need higher charge voltage per cell compared to Ni=
Cad.<br></div><div><br><a href=3D"http://www.ti.com/lit/an/snva557/snva557.=
pdf">http://www.ti.com/lit/an/snva557/snva557.pdf</a><br>&quot;...most Ni-C=
d cells will easily tolerate a sustained charging current of c/10 (1/10 oft=
he cell&#39;s A-hr rating) indefinitely with no damage to the cell.&quot;<b=
r>
&quot;Ni-MH cells are not as tolerant of sustained charging: the maximum sa=
fe trickle<br>charge rate will be specified by the manufacturer, and will p=
robably be somewhere<br>between c/40 and c/10&quot;<br><br></div>So yes, th=
ey&#39;re similar, but they&#39;re not the same and I wouldn&#39;t bet my l=
ife on that it doesn&#39;t matter.<br>
<br></div>I replace NiCads with Litium cells and modify the circuit to prev=
ent charge.<br></div>If I knew how to calculate the correct charge current =
for NiMH I might have chosen to modify the circuit for that instead.<br>
</div><br>I choose to play it safe.<br>Your choice is yours :)<br><br></div=
>Regards,<br>Peter<br><br></div><div class=3D"gmail_extra"><br><br><div cla=
ss=3D"gmail_quote">On 24 January 2014 12:38, Steve Lenham <span dir=3D"ltr"=
>&lt;<a href=3D"mailto:steve@bendentech.co.uk" target=3D"_blank">steve@bend=
entech.co.uk</a>&gt;</span> wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex">I have never encountered a synth with a NiCd=
/NiMH memory backup battery that does not simply trickle charge it with a s=
mall current - in that situation, you can use NiCd and NiMH interchangeably=
, and I do so regularly. It&#39;s only if you want to rapid-charge them tha=
t it matters.<br>

<br>
The OP wrote that it is a memory backup battery, so the current drain is ne=
gligible and the capacity uncritical - the battery is probably self-dischar=
ging faster than it being drained by the CMOS RAM device that it maintains.=
<br>

<br>
So the answer to the original question is that either would be OK, but if t=
he larger capacity device will physically still fit then there is no reason=
 not to go for that one.<br>
<br>
Cheers,<br>
<br>
Steve L.<br>
<br>
Benden Sound Technology<br>
<a href=3D"http://www.bendensound.co.uk" target=3D"_blank">www.bendensound.=
co.uk</a><div class=3D"im"><br>
<br>
<br>
On 23/01/2014 19:47, Peter M=F6rck wrote:<br>
</div><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-l=
eft:1px #ccc solid;padding-left:1ex"><div class=3D"im">
OK, get another NiMH then, should be no problem :)<br>
Old synths usually have NiCad and the recharging circuit probably isn&#39;t=
<br>
designed to handle NiMH (which has different charging characteristics).<br>
<br>
<br>
<br>
On 23 January 2014 20:42, M V &lt;<a href=3D"mailto:mv_synths@hotmail.com" =
target=3D"_blank">mv_synths@hotmail.com</a><br></div><div class=3D"im">
&lt;mailto:<a href=3D"mailto:mv_synths@hotmail.com" target=3D"_blank">mv_sy=
nths@hotmail.com</a>&gt;<u></u>&gt; wrote:<br>
<br>
=A0 =A0 Yes, it&#39;s an NIMH battery... on a pretty small synth.<br>
<br>
<br>
<br>
<br></div>
=A0 =A0 ------------------------------<u></u>------------------------------=
<u></u>------------<div class=3D"im"><br>
=A0 =A0 Date: Thu, 23 Jan 2014 20:40:24 +0100<br>
=A0 =A0 Subject: Re: [AH] Does mAh value matter on a memory backup battery?=
<br></div>
=A0 =A0 From: <a href=3D"mailto:peter.morck@gmail.com" target=3D"_blank">pe=
ter.morck@gmail.com</a> &lt;mailto:<a href=3D"mailto:peter.morck@gmail.com"=
 target=3D"_blank">peter.morck@gmail.com</a>&gt;<br>
=A0 =A0 To: <a href=3D"mailto:mv_synths@hotmail.com" target=3D"_blank">mv_s=
ynths@hotmail.com</a> &lt;mailto:<a href=3D"mailto:mv_synths@hotmail.com" t=
arget=3D"_blank">mv_synths@hotmail.com</a>&gt;<br>
=A0 =A0 CC: <a href=3D"mailto:analogue@hyperreal.org" target=3D"_blank">ana=
logue@hyperreal.org</a> &lt;mailto:<a href=3D"mailto:analogue@hyperreal.org=
" target=3D"_blank">analogue@hyperreal.org</a><u></u>&gt;<div class=3D"im">=
<br>
<br>
<br>
=A0 =A0 The big question is: Is it a rechargeable battery?<br>
=A0 =A0 100mAh sounds awfully low though, so I&#39;m guessing no.<br>
=A0 =A0 Is it a toy? :P<br>
<br>
=A0 =A0 Capacity is a measure of how much charge the battery has.<br>
=A0 =A0 100mAh will last for 100 hours if the synth drains 1mA from it, 100=
0<br>
=A0 =A0 hours if it drains 0.1mA from it, and so on... (in theory)<br>
=A0 =A0 So, bigger is better as it will last longer.<br>
<br>
=A0 =A0 /Peter<br>
<br>
<br>
<br>
<br>
<br>
=A0 =A0 On 23 January 2014 20:33, M V &lt;<a href=3D"mailto:mv_synths@hotma=
il.com" target=3D"_blank">mv_synths@hotmail.com</a><br></div><div class=3D"=
im">
=A0 =A0 &lt;mailto:<a href=3D"mailto:mv_synths@hotmail.com" target=3D"_blan=
k">mv_synths@hotmail.com</a>&gt;<u></u>&gt; wrote:<br>
<br>
=A0 =A0 =A0 =A0 I&#39;m looking to replace a memory backup battery on a syn=
th. The<br>
=A0 =A0 =A0 =A0 original was a 3.6V 100mAh battery, but the available mAh v=
alues<br>
=A0 =A0 =A0 =A0 for new ones are 80 and 150. I&#39;m guessing that such a s=
mall<br>
=A0 =A0 =A0 =A0 difference won&#39;t matter, but am unsure whether it&#39;s=
 better to<br>
=A0 =A0 =A0 =A0 shoot high or low.<br>
<br>
=A0 =A0 =A0 =A0 thanks,<br>
<br>
=A0 =A0 =A0 =A0 Matt<br>
<br>
<br>
<br>
<br>
</div></blockquote>
<br>
</blockquote></div><br></div>

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