Hi Ron,
The proper charging voltage is 13.8 V across the battery terminals (!). To
get this voltage I have to supply my K2 (sn 2894) with ~14.1 V from the
power supply because of the forward votage drop caused by the diode.
Looks like you either need a separate power supply for charging or even
better - an adjustable one. I am using Alinco DM-330MV which has an
adjustable "PRESET" function. My PRESET is set for 14.1 volts and there is a
little switch on the back of the PS which activates the preset voltage when
I need to charge the battery. If you supply K2 with exactly 13.8 volts at
the external power supply you risk damaging the battery by undercharging it
(!) especially if you run K2 at the same time which will cause the voltage
to drop further.
You won't overcharge the battery as long as the voltage across the battery
terminals does not exceed 13.8 Volts. The SLA battery will be in "float"
charge mode where the battery will self-regulate the charging current when
supplied with 13.5-13.8 volts. In this mode the charging current will equal
the self-discharge current and the battery can (float) stay indefinitely in
a fully charged state. Less than 13.5 volts is considered "undercharge"
which will decrease the capacity of the battery and will lead to "sulfation"
shortening its life. More than 13.8 V is charging mode (usually at around
14.1-14.5 until the current drops to a specific level indicating fully
charged battery) but if the battery is already fully charged, then it will
be overcharging if you maintain higher then 13.8 voltage. Both
"undercharging" and "overcharging" will damage the battery and severe
"overcharging" can damage even your K2. In overcharge mode the SLA battery
produces excessive amounts of hydrogen and as the pressure builds up it is
vented from a special relief valve, along with corrosive fumes from the acid
and the battery dries out. When severely overcharged, the battery also heats
up and as it heats up will accept more current and this will make it event
hotter - it's called thermal runaway and the battery will be permanently
damaged and may lead to damage of your K2 - not a pretty sight.
If you have an adjustable power supply you can connect a DMM across the
battery terminals and start slowly increasing the voltage until your DMM
reads 13.8 V. Then disconnect the DMM from the battery and connect it to the
power supply and note the voltage coming from the power supply. Maintain
this voltage when K2 is not in use. This will keep the battery fully charged
at all times.
If you are using the radio and you are transmitting for prolonged periods of
time it is better to check how much the voltage drops across the battery
terminals when transmitting at full power with K2 and the radio is supplied
with the voltage necessary to maintain 13.8 v at the battery terminals. If
at full power the voltage does not drop under 13.5 volts you don't have to
turn the battery switch off.
You MUST turn the battery switch off when you power your radio from an
external 12 V battery - for example when mobile with the engine not running.
What I usually do is to charge my battery for few days when not using K2 -
every 2-3 weeks, ill leave my battery on "float" for 2-3 days and then turn
the battery switch off and use my K2 as usual.
73s
Andrey E. Stoev
KB1FZA
----- Original Message -----
From: "Ron McDowell - W5RCM / ZS1MCD" <[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Thursday, May 19, 2005 10:20 PM
Subject: [Elecraft] KBT2 charging
According to the builtin volt meter on my K2, my old Astron RS-20A is
putting out 13.3V (which I understand to be really more like 13.5-13.6
because of voltage drop across the diode). A couple questions regarding
that and the KBT2:
1. Is that adequate voltage for charging the KBT2? The battery manual
calls for 13.8V.
2. Is it OK to leave the red battery switch on the back of the rig ON
whenever I'm using the K2 on the power supply...in other words, am I
running any risk of overcharging the battery? Or should I switch the
battery off unless I'm running from (or charging up) the battery?
Thanks for the input!
--
72, Ron - W5RCM / ZS1MCD
Austin TX / Hermanus ZA
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